## What kind of change does this PR introduce?
Chore that resolves FE-2785.
## What is the current behavior?
The Log Drains headers section still uses a bespoke add-header mini form
and stores headers directly as a record in form state.
That makes it inconsistent with the shared `KeyValueFieldArray` pattern
already adopted elsewhere in Studio.
## What is the new behavior?
- reuses the shared `KeyValueFieldArray` in the Log Drains destination
sheet
- keeps the external submit contract unchanged by converting between:
- form-only `headerEntries: { key, value }[]`
- submitted `headers: Record<string, string>`
- preserves type-specific defaults:
- webhook starts with `Content-Type: application/json`
- OTLP starts with `Content-Type: application/x-protobuf`
- moves header validation into standard form errors for:
- max 20 headers
- duplicate header names
- partially filled rows, while still allowing fully empty draft rows
- adds focused utility and sheet tests for the new adapter and
validation behaviour
| Before | After |
| --- | --- |
| <img width="1728" height="997" alt="Log Drains Settings Mallet
Toolshed Supabase-027E4669-8B02-4C43-8771-794E13799FA3"
src="https://github.com/user-attachments/assets/43ed6334-28ef-4d47-9747-dfb3221462ec"
/> | <img width="1728" height="997" alt="Log Drains Settings Mallet
Toolshed Supabase-68477EA1-6F56-4BE2-9355-C121896F11E4"
src="https://github.com/user-attachments/assets/9391bccd-3a50-4468-91e7-05059d41543c"
/> |
## Additional context
This is PR 4 of the DEPR-394 field-array stack.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
## Release Notes
* **Bug Fixes**
* Enhanced header validation with specific error messages for duplicate
headers and missing values.
* Improved form behavior for edge cases in header entry management.
* **New Features**
* Added destination-specific default headers for clearer initial
configuration.
* **Tests**
* Added comprehensive test suite validating header management and form
submission behavior.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Charis