chore: implements error handling tests for fastapi

[dom96]

simple routes which perform validation using pydantic and which return exceptions/other common error status codes

[ask-bonk]

I've reviewed the full diff, the existing test patterns, and the shared client helpers. The PR adds error-handling test routes to the FastAPI worker and a corresponding test file. The code is clean, follows existing conventions in the test suite, and the tests cover meaningful scenarios (HTTPException variants, Pydantic validation for body/query/path, custom exception handlers, and unhandled exceptions).

One minor issue: the error_validate_body route returns only name and price but omits quantity, even though quantity is part of the ItemModel. The test test_valid_body_passes only checks name and price so it passes, but the route silently drops a field from the model. This isn't a bug per se (the test does pass), but it's inconsistent — a test validating the happy path should ideally verify all accepted fields round-trip correctly. However, this is a test file, and the route is purpose-built for testing validation, so this is borderline nitpick territory.

I see no logic bugs, security issues, or other actionable problems. The tests are well-structured, use the established _client helpers, follow the pytest.mark.asyncio + fastapi_app fixture pattern from the other test files, and cover the cases described in the module docstring.

LGTM

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