Fix AttributeError on shutdown when threading.Thread is replaced mid-tracking

[swaroski]

Closes #856

Summary

• Tracker.__enter__ instruments threading.Thread by setting _name and _ident descriptors on the class.
• If the tracked script replaces threading.Thread during tracking (notably gevent.monkey.patch_all()),
  Tracker.__exit__ previously looked up threading.Thread again and tried to delattr from a different class,
  raising AttributeError.
• Now the patched class is captured on entry and cleaned up on exit, with a defensive AttributeError suppression.

Test plan

• New regression test in tests/unit/test_tracker.py swaps threading.Thread mid-context.
• Reproduced the original gevent crash from the issue, confirmed fixed against the rebuilt extension.

[godlygeek]

Looks reasonable. Like the other PRs, should only have the last commit, and needs a Signed-off-by line with your real name in it.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.48%. Comparing base (015e6f0) to head (924935d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #910      +/-   ##
==========================================
+ Coverage   92.46%   92.48%   +0.02%     
==========================================
  Files          99       99              
  Lines       11748    11754       +6     
  Branches      427      427              
==========================================
+ Hits        10863    10871       +8     
+ Misses        885      883       -2     

Flag	Coverage Δ
cpp	92.48% <100.00%> (+0.02%)	⬆️
python_and_cython	92.48% <100.00%> (+0.02%)	⬆️

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[swaroski]

@godlygeek addressed: switched test to monkeypatch fixture, added
Signed-off-by with full name.

[godlygeek]

LGTM