Python 3.9 reached end-of-life and the codebase uses PEP 604 union
type syntax (str | None) which requires Python 3.10+.
Changes:
- Remove Python 3.9 from CI build matrix
- Update requires-python to >=3.10 in all pyproject.toml files
- Update classifiers to reflect supported Python versions (3.10-3.13)
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