Replace request.param fixture selection with typed parameters

[adamtheturtle]

Report month, image source, and file mode selection currently depend on parametrized fixtures reading request.param. Replace them with ordinary typed test parameters and a shared image factory fixture that owns file handles through ExitStack.

Retain all 467 collected case IDs and every existing assertion, including the buffer cases repeated for both file modes. Keep fixture annotations importable at runtime for pytest-beartype, and check the factory's source, mode, and return type with Beartype.

Validation: final full suites on Python 3.14 and final Python 3.15 each pass all 467 cases with 100% branch coverage; all four type checkers and public type completeness; applicable pre-commit hooks and Pylint; direct checks for image bytes, file cleanup, buffer ownership, and invalid source/mode rejection. Local Mypy used MYPY_NUM_WORKERS=0.

Existing limitation: selecting only README.rst and docs/source/index.rst enables package import checking and fails three examples because vws.reports._checked annotates hint as type[T] while receiving a generic alias. The same failure reproduces on unchanged main. The repository's complete test collection passes its documentation examples; its existing package-check configuration remains enabled.