Add CI for supported Python versions and package builds

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Problem

The package advertises Python 3.9 through 3.12 support but has no automated workflow proving that the tests, package metadata, and built distributions work across those versions.

Proposed work

• Add a GitHub Actions matrix for Python 3.9, 3.10, 3.11, and 3.12.
• Run the complete test suite and coverage reporting.
• Build the source distribution and wheel.
• Install the built wheel in a clean environment and run an import/parser smoke test.
• Resolve any mismatch between declared and actual Python compatibility.

Acceptance criteria

• Every advertised Python version passes in CI.
• Overall coverage remains at or above 85%.
• Source and wheel builds succeed without undeclared files.
• The built wheel imports and parses a representative response.
• Pull requests display required test and build status.