chore: support python 3.15

[WilliamBergamin]

Summary

This pull request adds Python 3.15 to the supported versions now that Python 3.15.0 is out.

• Adds 3.15 to the unit test matrix in ci-build.yml, keeping every existing entry (PyPy included).
• Sets LATEST_SUPPORTED_PY to 3.15 in ci-build.yml, so lint, typecheck, and coverage run on 3.15.
• Adds the Programming Language :: Python :: 3.15 classifier to pyproject.toml.

Testing

• Confirm the new 3.15 unit test job appears and passes in this PR's checks.
• Confirm the lint, typecheck, and code coverage jobs now run on Python 3.15 and pass.
• Ran ./scripts/install_and_run_tests.sh on a local Python 3.15.0 venv: passed (format, lint, mypy clean; 50 passed).

Special notes

None.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/install_and_run_tests.sh after making the changes.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.68%. Comparing base (b721526) to head (9699340).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #144   +/-   ##
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  Coverage   94.68%   94.68%           
=======================================
  Files          11       11           
  Lines         207      207           
=======================================
  Hits          196      196           
  Misses         11       11           

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