[Daxia] [ FastAPI ] Add FastAPITestClient auth helpers

[daxia778]

/claim #804

Summary

• Add an opt-in FastAPITestClient subclass while keeping TestClient as the unchanged Starlette export.
• Add bearer and basic authentication helpers plus reset_auth() for clearing auth state.
• Add ws_connect() for WebSocket tests with custom headers and subprotocols.
• Add assert_status() to issue a request and raise a helpful assertion message when the status differs.

Demo

https://github.com/daxia778/Bounty-Hunters/releases/download/issue-804-demo-d87bb4f9/fastapi-testclient-804-demo-d87bb4f9.mp4

Validation

• UV_PYTHON=<Python 3.12> uv run python -m pytest tests/test_fastapi_testclient.py -q
• UV_PYTHON=<Python 3.12> uv run ruff check fastapi/testclient.py tests/test_fastapi_testclient.py
• UV_PYTHON=<Python 3.12> uv run ruff format --check fastapi/testclient.py tests/test_fastapi_testclient.py
• git diff --check

[github-actions]

Unfortunately the changes in this PR didn't fully resolve the issue. Please rework your solution and submit a new pull request.

Make sure to review the acceptance criteria in the linked issue and verify all conditions are met before resubmitting. See CONTRIBUTING.md for guidelines.