CI Python Versions: Update to Python 3.15 #26281

Python 3.15 has a published release candidate according to the Python.org downloads API, but is missing from at least one CI test matrix.

Requested changes

• Ensure every explicit-version python matrix in .github/workflows/pr-check.yml includes Python 3.15.
• Ensure every explicit-version python matrix in .github/workflows/build.yml includes Python 3.15.
• Run the tests on Python 3.15 and resolve any compatibility failures.

Compare matrix entries by major.minor; patch and prerelease/dev variants of 3.15 already count as coverage. Add the version only where missing and preserve existing entries. Floating 3.x entries do not declare release-candidate coverage.

Keep minimum-supported-version and build-runtime Python pins unchanged; adding CI coverage does not require raising the minimum supported Python version.