[aryan maheshwari (arymaheshwari)]

Fixes #26281

Add Python 3.15 to the explicit-version Python test matrices in .github/workflows/pr-check.yml and .github/workflows/build.yml. Existing matrix entries, floating 3.x jobs, and Python 3.10 build/runtime pins are preserved.

Validation:

• Ubuntu through WSL, CPython 3.15.2, pytest 8.4.2: 226 passed, 2 skipped.
• Ubuntu through WSL, CPython 3.15.2, pip install --upgrade --pre pytest equivalent (resolved to pytest 9.2.0): 226 passed, 2 skipped. One existing third-party pytest-json deprecation warning.
• Full Python suite run with python python_files/tests/run_all.py -q --timeout=90 using the repository's base and test requirements.
• Windows, CPython 3.15.2, both pytest configurations: 220 passed, 6 skipped, 2 failures caused by the local account lacking symlink privileges (WinError 1314). The same symlink permission error occurs with Python 3.13; the symlink tests pass on Ubuntu.
• Both workflow files parse as YAML and pass Prettier. Matrix checks verify that only the missing 3.15 entries were added and existing entries/runtime pins are preserved.

No Python compatibility code changes were needed.

[Eduardo Villalpando Mello (edvilme)]

LGTM, thank you very much for taking on this!
I will let the CI run so we can merge :)