Restore notebook diff preview for completed updates (#49402)

## Summary

This is **PR 3 of 3** in the FE-4243 stack fixing "Notebook update
proposal shows 'unapplyable' error for already-completed updates."

- Consumes the `previous_content` field added by PR 2 (#49401) to
reconstruct diffs for already-applied notebook updates
- Restores the diff preview that PR 1 initially dropped — completed
updates now show the full before/after instead of a generic "Notebook
updated" message
- Uses the same diff derivation function called pre-approval,
guaranteeing the rendered diff matches what was shown during
confirmation
- Includes defensive fallback handling for older persisted chats (before
`previous_content` existed) and edge cases

**Depends on**: PR 2 (#49401) merging first — this PR consumes the
`previous_content` field from that server change.

Resolves FE-4243 

## Test plan

- ✅ 19/19 tests pass in NotebookProposalRenderer.test.tsx (2 confirmed
as real regressions)
- ✅ 118/118 tests pass in full AIAssistantPanel suite
- ✅ Typecheck: clean on modified files
- ✅ ESLint: zero errors/warnings on changed files  
- ✅ Lint ratchet: passes (some rules improved)
- ✅ New regression tests cover: delete_cell, insert_cell, missing
previous_content, and operations that no longer reconcile
- ✅ No notebook fetch in completed update tests (proves no redundant
re-fetching)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added visual previews showing notebook changes, including inserted and
deleted cells, when prior content is available.
  * Prevented duplicate cells from appearing in update previews.
* Retained a compact completion message when change details are
unavailable or inconsistent.
  * Ensured previews are shown only for the relevant notebook.

* **Tests**
* Added coverage for notebook update previews, deletion and insertion
diffs, duplicate prevention, notebook matching, and fallback behavior.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/studio/components/ui/AIAssistantPanel/Message.utils.ts

@@ -2,7 +2,7 @@ import { untrustedSql } from '@supabase/pg-meta'
 import { z, type SafeParseReturnType } from 'zod'
 
 import { notebookOperationsSchema } from '@/data/content/notebooks/notebook-operations'
-import { agentNotebookSchema } from '@/data/content/notebooks/notebook-schema'
+import { agentNotebookSchema, notebookSchema } from '@/data/content/notebooks/notebook-schema'
 
 // Splits markdown into alternating [plain, code, plain, code, ...] segments.
 // Odd-indexed segments are already inside code spans/fences and should be left alone.
@@ -143,6 +143,10 @@ export const updateNotebookInputSchema = z.object({
 
 export const notebookToolOutputSchema = z.object({ id: z.string(), name: z.string() })
 
+export const updateNotebookToolOutputSchema = notebookToolOutputSchema.extend({
+  previous_content: notebookSchema.optional(),
+})
+
 export const rateMessageResponseSchema = z.object({
   category: z.enum([
     'sql_generation',

apps/studio/components/ui/AIAssistantPanel/NotebookProposalRenderer.test.tsx

@@ -339,6 +339,141 @@ describe('NotebookProposalRenderer', () => {
     expect(screen.queryByText("This update can't be applied as written")).not.toBeInTheDocument()
   })
 
+  it('renders the snapshot-derived diff for a completed delete_cell update, without fetching the notebook', () => {
+    render(
+      <NotebookProposalRenderer
+        mode="update"
+        state="output-available"
+        input={{
+          id: NOTEBOOK_ID,
+          expected_updated_at: '2024-01-01T00:00:00.000Z',
+          operations: [{ _tag: 'delete_cell', cell_id: 'cell-1' }],
+        }}
+        output={{
+          id: NOTEBOOK_ID,
+          name: 'Signup funnel',
+          previous_content: {
+            schema_version: 1,
+            cells: [
+              { _tag: 'markdown_cell', _id: 'cell-1', text: 'hello' },
+              { _tag: 'markdown_cell', _id: 'cell-2', text: 'world' },
+            ],
+          },
+        }}
+      />
+    )
+
+    expect(screen.getByText('−1')).toBeInTheDocument()
+    expect(screen.getByRole('link', { name: 'Open notebook' })).toHaveAttribute(
+      'href',
+      `/project/default/explorer/notebook/${NOTEBOOK_ID}`
+    )
+  })
+
+  it('renders a single added cell for a completed insert_cell update, not a phantom duplicate', () => {
+    render(
+      <NotebookProposalRenderer
+        mode="update"
+        state="output-available"
+        input={{
+          id: NOTEBOOK_ID,
+          expected_updated_at: '2024-01-01T00:00:00.000Z',
+          operations: [
+            {
+              _tag: 'insert_cell',
+              after_cell_id: 'cell-1',
+              cell: { _tag: 'markdown_cell', text: 'new section' },
+            },
+          ],
+        }}
+        output={{
+          id: NOTEBOOK_ID,
+          name: 'Signup funnel',
+          previous_content: {
+            schema_version: 1,
+            cells: [{ _tag: 'markdown_cell', _id: 'cell-1', text: 'hello' }],
+          },
+        }}
+      />
+    )
+
+    expect(screen.getByText('+1')).toBeInTheDocument()
+    expect(screen.getAllByRole('button', { name: 'Added Markdown cell' })).toHaveLength(1)
+  })
+
+  it('falls back to the compact completed body when previous_content is absent', () => {
+    render(
+      <NotebookProposalRenderer
+        mode="update"
+        state="output-available"
+        input={{
+          id: NOTEBOOK_ID,
+          expected_updated_at: '2024-01-01T00:00:00.000Z',
+          operations: [{ _tag: 'delete_cell', cell_id: 'cell-1' }],
+        }}
+        output={{ id: NOTEBOOK_ID, name: 'Signup funnel' }}
+      />
+    )
+
+    expect(screen.getByText('Notebook updated: Signup funnel')).toBeInTheDocument()
+    expect(screen.queryByText("This update can't be applied as written")).not.toBeInTheDocument()
+    expect(screen.getByRole('link', { name: 'Open notebook' })).toBeInTheDocument()
+  })
+
+  it('falls back to the compact completed body when the snapshot no longer matches the operations', () => {
+    render(
+      <NotebookProposalRenderer
+        mode="update"
+        state="output-available"
+        input={{
+          id: NOTEBOOK_ID,
+          expected_updated_at: '2024-01-01T00:00:00.000Z',
+          operations: [{ _tag: 'delete_cell', cell_id: 'missing' }],
+        }}
+        output={{
+          id: NOTEBOOK_ID,
+          name: 'Signup funnel',
+          previous_content: {
+            schema_version: 1,
+            cells: [{ _tag: 'markdown_cell', _id: 'cell-1', text: 'hello' }],
+          },
+        }}
+      />
+    )
+
+    expect(screen.getByText('Notebook updated: Signup funnel')).toBeInTheDocument()
+    expect(screen.queryByText("This update can't be applied as written")).not.toBeInTheDocument()
+    expect(screen.queryByText('Preview unavailable')).not.toBeInTheDocument()
+  })
+
+  it('falls back to the compact completed body when the output id does not match the requested notebook', () => {
+    render(
+      <NotebookProposalRenderer
+        mode="update"
+        state="output-available"
+        input={{
+          id: NOTEBOOK_ID,
+          expected_updated_at: '2024-01-01T00:00:00.000Z',
+          operations: [{ _tag: 'delete_cell', cell_id: 'cell-1' }],
+        }}
+        output={{
+          id: 'some-other-notebook-id',
+          name: 'Signup funnel',
+          previous_content: {
+            schema_version: 1,
+            cells: [
+              { _tag: 'markdown_cell', _id: 'cell-1', text: 'hello' },
+              { _tag: 'markdown_cell', _id: 'cell-2', text: 'world' },
+            ],
+          },
+        }}
+      />
+    )
+
+    expect(screen.getByText('Notebook updated: Signup funnel')).toBeInTheDocument()
+    expect(screen.queryByText('−1')).not.toBeInTheDocument()
+  })
+
   it('derives the diff against live content for a denied update', async () => {
     mockContentItem(mockNotebookRow())
 

apps/studio/components/ui/AIAssistantPanel/NotebookProposalRenderer.tsx

@@ -13,6 +13,7 @@ import {
   createNotebookInputSchema,
   notebookToolOutputSchema,
   updateNotebookInputSchema,
+  updateNotebookToolOutputSchema,
 } from './Message.utils'
 import { AlertError } from '@/components/ui/AlertError'
 import {
@@ -340,8 +341,31 @@ function UpdateNotebookProposal({
   }
 
   if (isCompleted) {
-    const parsedOutput = notebookToolOutputSchema.safeParse(output)
+    const parsedOutput = updateNotebookToolOutputSchema.safeParse(output)
     const notebookName = parsedOutput.success ? parsedOutput.data.name : undefined
+    const isOutputForRequestedNotebook =
+      parsedOutput.success && parsedOutput.data.id === parsedInput.data.id
+    const previousContent = isOutputForRequestedNotebook
+      ? parsedOutput.data.previous_content
+      : undefined
+    const diff = previousContent
+      ? deriveNotebookDiff(previousContent, parsedInput.data.operations)
+      : undefined
+
+    if (diff?.success) {
+      return (
+        <NotebookConfirm
+          mode="update"
+          confirmState={confirmState}
+          footerAction={footerAction}
+          message={`Assistant wants to update "${notebookName}"`}
+          onApprove={onApprove}
+          onDeny={onDeny}
+        >
+          <AssistantNotebookPreview entries={diff.entries} mode="update" title={notebookName} />
+        </NotebookConfirm>
+      )
+    }
 
     return (
       <NotebookConfirm