Add OpenAPI examples for apis

[Emoji-dot]

close #643
Implemented the API marketplace OpenAPI example work and kept it scoped to spec/docs plus a focused regression test. The updated files are the OpenAPI spec, the new API OpenAPI example test, and the contract testing note.

What changed
Added request/response examples for GET /api/apis
Added request/response examples for POST /api/apis
Added response examples for GET /api/apis/{id}
Added request/response examples for POST /api/apis/{id}/endpoints/bulk
Documented that API marketplace OpenAPI examples now have regression coverage
There are no runtime API behavior changes here. This is a documentation/spec improvement only.

Checks
npm run test:serial -- src/routes/apis.openapi.test.ts passed
npx eslint 'src/routes/apis.openapi.test.ts' passed
Note
When I ran a broader OpenAPI example suite earlier, an existing unrelated failure showed up in src/routes/billing.openapi.test.ts because the billing spec is missing a 429 example. I did not change that endpoint as part of this task.

Review note
Everything requested here is implemented except Code review approved, which is an external workflow step rather than something I can complete inside the repo.

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