Exercise FastAPI endpoints through TestClient and mock FreshRSS transport

[Skulldorom]

Motivation

• Provide HTTP-level tests for the API so endpoints are validated through a TestClient instead of only calling functions directly.
• Keep the FreshRSS transport boundary mocked while preserving focused unit tests for token parsing to make failures easier to diagnose.
• Ensure category labels are safely encoded when placed into the upstream FreshRSS stream path.

Description

• Add a reusable main_module fixture that imports the application under deterministic FRESHRSS_* environment variables and a test_client fixture that yields a TestClient for HTTP requests (tests/test_main.py).
• Introduce install_freshrss_transport to stub only requests.post and requests.get, leaving the HTTP surface untouched and capturing outbound calls for assertions (tests/test_main.py).
• Add HTTP-level tests for /health and /freshrss/unread covering default and explicit n and category parameters, invalid n values (validation), encoded category values, authentication caching (login once), upstream 502 serialization, and response JSON shape (tests/test_main.py).
• Retain focused unit tests that exercise get_greader_token failure cases (failed login and missing Auth= line) while mocking only the transport (tests/test_main.py).
• Percent-encode FreshRSS category labels before embedding them in the upstream stream path by importing and using quote from urllib.parse in main.py.

Testing

• Ran python -m py_compile main.py tests/test_main.py which succeeded.
• Ran git diff --check which found no issues during the change validation step.
• Attempted uv run pytest -q but test collection could not start because starlette.testclient requires the optional httpx2 package and the environment could not fetch packages from PyPI, so pytest could not be executed here.
• Attempted to add httpx2 to dev dependencies with the runtime tooling, but package installation failed due to lack of network access, so full test execution was not possible in this environment.

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[chatgpt-codex-connector]

Add TestClient's required transport dependency

With the dependencies installed by .github/workflows/docker.yml via uv sync --frozen, this import fails during test collection because Starlette's TestClient requires the httpx2 package, while the locked dev dependencies include only pytest. I reproduced the resulting RuntimeError with .venv/bin/pytest -q; therefore every CI test run fails before executing a test, and the dependent Docker publishing job is blocked. Add the required TestClient dependency to pyproject.toml and regenerate uv.lock.

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[Skulldorom]

Superseded by #25 — all four PRs merged with conflict resolution and review fixes applied.