ci: add step to check wheel contents before uploading artifacts

[tlambert03]

• Refactored stub generation in meson.build: The stub generation step now uses custom_target with explicit input/output, installs the stub directly to the correct site-packages directory, and adds a development-time copy to the source tree for Pyright compatibility. Pure-Python package files are now installed using py.install_sources, replacing previous manual install steps.

• Improved Windows DLL discovery for stub generation: Added _windows_dll_dirs context manager in scripts/build_stubs.py to ensure dependent DLLs are discoverable when importing the extension on Windows, preventing import errors during stub generation. [1] [2]

• Added wheel content validation to CI: The check-wheel-contents tool is now run in the GitHub Actions workflow to verify wheel correctness after building.

• Integrated wheel content checks in local test workflow: The pyproject.toml now requires check-wheel-contents for tests and runs it before pytest to ensure wheels are valid before running tests.

• Added configuration for check-wheel-contents: The pyproject.toml includes a [tool.check-wheel-contents] section specifying the package layout and ignored warnings for more robust wheel validation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.19%. Comparing base (ad263c7) to head (cc41ef6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #66   +/-   ##
=======================================
  Coverage   76.19%   76.19%           
=======================================
  Files           2        2           
  Lines        1218     1218           
  Branches      947      947           
=======================================
  Hits          928      928           
  Misses         92       92           
  Partials      198      198           

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