ci: Run common test suite on Python 3.14t

[alexander-alderman-webb]

Description

Noteworthy:

• Change to split_tox_gh_actions.py to support the free-threading t suffix in versions; and
• upgrade pip in tox for free-threaded Python by setting VIRTUALENV_PIP; and
• check the new 3.14 variable that controls if context is already propagated when creating threads in the threading tests. See https://docs.python.org/3/howto/free-threading-python.html#context-variables

Issues

Closes#4970

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.96%. Comparing base (2fadc27) to head (2bfc6bc).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@ Coverage Diff @@## master #4969 +/- ## ========================================== - Coverage 84.03% 83.96% -0.08%  ========================================== Files 165 165 Lines 17079 17079 Branches 3000 3000 ========================================== - Hits 14352 14340 -12 - Misses 1831 1840 +9 - Partials 896 899 +3 

see 4 files with indirect coverage changes

[sentrivana]

Awesome!