chore: adds more fastapi threading tests

[dom96]

Just a few more test cases which end up creating a thread via starlette:

• Sync routes
• Streaming
• File upload

Follow up to #190

[ask-bonk]

I've reviewed all the changes thoroughly. The code is clean test infrastructure. The only minor observation is that post_json is defined but never used, and build_multipart has a subtle issue with binary content handling. However, the post_json helper is reasonable to add proactively for future tests, and the build_multipart function works correctly for the test cases at hand.

The tests are well-structured, cover the documented call-sites for anyio.to_thread.run_sync, and the assertions are meaningful. The python-multipart dependency addition is correct — it's required by FastAPI for file upload handling. No security, correctness, or compatibility issues.

LGTM

github run

[ryanking13]

Nice!