PYTHON-6077 Officially support Python 3.15

[blink1073]

PYTHON-6077

Changes in this PR

• Added Python 3.15 (standard and free-threaded) to the Evergreen test matrix.
• Added a fetch fallback for Python 3.15 builds from python-build-standalone, since the Evergreen uv (0.8.24) has no 3.15 download and the toolchain does not provision it.
• Made task version selection always use UV_PYTHON.
• Preferred toolchain-installed pythons over uv-managed downloads.
• Added toolchain python directories to PATH.
• Bumped cibuildwheel to 4.x to build cp315 and cp315t wheels.
• Added the Python 3.15 classifier and a changelog entry.

Test Plan

• Ran just lint (includes the generate-config check).
• Ran just typing (mypy and pyright pass).
• Ran an Evergreen patch; Python 3.15 tasks failed until the fetch fallback was restored, then the fallback path resolved.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage? Added Evergreen CI coverage for the new versions.
• Is any followup work tracked in a JIRA ticket? If so, add link(s). Drop the python-build-standalone fallback once uv and the toolchain provide 3.15.

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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