## 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 -->
Charis
233cbdc8e5