Align outbound fallback expectations with the reviewed Responses id shape

The last CI failure after the review cleanup was only a stale test expectation:
function_call fallback items now correctly use fc_* item ids while preserving the
call_* id in call_id. Updating the test keeps the suite aligned with the
Responses payload shape we intentionally adopted in the bridge.

Constraint: Keep the fix to the assertion only; the production behavior already matches the reviewed contract
Rejected: Revert the fc_* item id change | that would reopen the review thread we intentionally fixed
Confidence: high
Scope-risk: narrow
Directive: When bridge output semantics change intentionally, update sibling facade tests in the same PR so CI reflects the new contract immediately
Tested: Focused vitest for openai responses outbound/response bridge, Gemini request bridge, Settings payload rules; typecheck; git diff --check
Not-tested: Full npm test

src/server/transformers/openai/responses/outbound.test.ts

@@ -244,7 +244,7 @@ describe('serializeResponsesFinalPayload', () => {
 
     expect(payload.output).toEqual([
       {
-        id: 'call_1',
+        id: 'fc_1',
         type: 'function_call',
         status: 'completed',
         call_id: 'call_1',