fix(studio): duplicate Content-Type on webhook log drains (#46673)

## Problem

Webhook log drains (project and org/audit) deliver requests with a
malformed, duplicated `Content-Type: application/jsonapplication/json`
header. On at least some receivers this breaks body parsing, so the
delivered body appears empty even though it is present (confirmed with
gzip both on and off).

Root cause: the create form seeds a default `Content-Type:
application/json` header for webhook drains, and the logflare webhook
adaptor's `Tesla.Middleware.JSON` also sets `content-type:
application/json` when it encodes the body. Both are sent, and the
receiver concatenates the two same-named headers.

## Fix

Stop seeding `Content-Type` in the webhook default headers
(`getDefaultHeadersByType`). The delivery side already sets it, so a
single clean header is sent. OTLP keeps its `application/x-protobuf`
default because the OTLP delivery path uses `json: false` and does not
set a content type itself.

Updated the form tests that assumed the seeded header (the added-header
row is now index 0 instead of 1, and the duplicate-header test now adds
two explicit rows).

## How to test

- Create a webhook (Custom Endpoint) audit log drain pointing at a
request bin.
- Trigger an audit event and inspect the delivered request:
`Content-Type` should be a single `application/json`, and the JSON body
should be visible.

## Note

This fixes the common case (the seeded default). A user who manually
adds a `Content-Type` header to a webhook drain would still hit the
duplication; the robust cross-team fix would be for the logflare webhook
adaptor to drop an incoming `content-type` before its JSON middleware
sets one. Flagging for the logs team.

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

* **Bug Fixes**
* Log Drain header handling corrected: webhook drains no longer add a
default Content-Type; other drain types retain their appropriate
defaults. Empty header rows are no longer submitted.
* **Tests**
* Updated tests to match new header indexing, validation behavior, and
submission expectations.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

apps/studio/components/interfaces/LogDrains/LogDrainDestinationSheetForm.test.tsx

@@ -85,13 +85,13 @@ describe('LogDrainDestinationSheetForm', () => {
     useTrackMock.mockReturnValue(trackMock)
   })
 
-  it('shows the JSON content type header for webhook create mode', async () => {
+  it('does not prefill a Content-Type header for webhook create mode', async () => {
     renderForm()
 
     await screen.findByRole('dialog')
 
-    expect(screen.getByDisplayValue('Content-Type')).toBeInTheDocument()
-    expect(screen.getByDisplayValue('application/json')).toBeInTheDocument()
+    expect(screen.queryByDisplayValue('Content-Type')).not.toBeInTheDocument()
+    expect(screen.queryByDisplayValue('application/json')).not.toBeInTheDocument()
   })
 
   it('wraps the save button with the save destination shortcut while open', async () => {
@@ -169,8 +169,8 @@ describe('LogDrainDestinationSheetForm', () => {
     const headerNameInputs = screen.getAllByPlaceholderText('Header name')
     const headerValueInputs = screen.getAllByPlaceholderText('Header value')
 
-    await user.type(headerNameInputs[1], 'X-API-Key')
-    await user.type(headerValueInputs[1], 'secret-key')
+    await user.type(headerNameInputs[0], 'X-API-Key')
+    await user.type(headerValueInputs[0], 'secret-key')
 
     submitForm()
 
@@ -179,7 +179,6 @@ describe('LogDrainDestinationSheetForm', () => {
       expect.objectContaining({
         type: 'webhook',
         headers: {
-          'Content-Type': 'application/json',
           'X-API-Key': 'secret-key',
         },
       })
@@ -199,12 +198,15 @@ describe('LogDrainDestinationSheetForm', () => {
       'https://logs.example.com/ingest'
     )
     await user.click(screen.getByRole('button', { name: 'Add a new header' }))
+    await user.click(screen.getByRole('button', { name: 'Add a new header' }))
 
     const headerNameInputs = screen.getAllByPlaceholderText('Header name')
     const headerValueInputs = screen.getAllByPlaceholderText('Header value')
 
-    await user.type(headerNameInputs[1], 'Content-Type')
-    await user.type(headerValueInputs[1], 'application/custom')
+    await user.type(headerNameInputs[0], 'X-Custom')
+    await user.type(headerValueInputs[0], 'one')
+    await user.type(headerNameInputs[1], 'X-Custom')
+    await user.type(headerValueInputs[1], 'two')
 
     submitForm()
 
@@ -231,11 +233,9 @@ describe('LogDrainDestinationSheetForm', () => {
     expect(onSubmit).toHaveBeenCalledWith(
       expect.objectContaining({
         type: 'webhook',
-        headers: {
-          'Content-Type': 'application/json',
-        },
       })
     )
+    expect(onSubmit.mock.calls[0][0]).not.toHaveProperty('headers')
     expect(screen.queryByText('undefined')).not.toBeInTheDocument()
   })
 
@@ -253,7 +253,7 @@ describe('LogDrainDestinationSheetForm', () => {
     await user.click(screen.getByRole('button', { name: 'Add a new header' }))
 
     const headerNameInputs = screen.getAllByPlaceholderText('Header name')
-    await user.type(headerNameInputs[1], 'X-Draft-Only')
+    await user.type(headerNameInputs[0], 'X-Draft-Only')
 
     submitForm()
 
@@ -276,7 +276,7 @@ describe('LogDrainDestinationSheetForm', () => {
     await user.click(screen.getByRole('button', { name: 'Add a new header' }))
 
     const headerValueInputs = screen.getAllByPlaceholderText('Header value')
-    await user.type(headerValueInputs[1], 'draft-value')
+    await user.type(headerValueInputs[0], 'draft-value')
 
     submitForm()
 

apps/studio/components/interfaces/LogDrains/LogDrains.utils.test.ts

@@ -36,10 +36,8 @@ describe('getHeadersSectionDescription', () => {
 })
 
 describe('getDefaultHeadersByType', () => {
-  it('returns the JSON content type header for webhook destinations', () => {
-    expect(getDefaultHeadersByType('webhook')).toEqual({
-      'Content-Type': 'application/json',
-    })
+  it('does not return a default Content-Type header for webhook destinations', () => {
+    expect(getDefaultHeadersByType('webhook')).toEqual({})
   })
 
   it('returns the protobuf content type header for OTLP destinations', () => {

apps/studio/components/interfaces/LogDrains/LogDrains.utils.ts

@@ -40,8 +40,10 @@ export const HEADER_VALIDATION_ERRORS = {
   VALUE_REQUIRED: 'Header value is required',
 } as const
 
+// Webhook drains intentionally omit a Content-Type default: the delivery side already sets
+// `application/json`, and seeding it here produces a duplicated Content-Type header that can
+// break body parsing on the receiver. OTLP needs it since its delivery does not set one.
 const DEFAULT_HEADERS_BY_TYPE: Partial<Record<LogDrainType, Record<string, string>>> = {
-  webhook: { 'Content-Type': 'application/json' },
   otlp: { 'Content-Type': 'application/x-protobuf' },
 }