fix(profiler): Stop nulling buffer on teardown

[ericapisani]

There's still the occasional race condition in the GEvent tests that may be related to fast "start/stop" cycles, and the teardown functions were not updated as part of #5622 (comment)

These changes remove self.buffer = None from teardown() in both ThreadContinuousScheduler and GeventContinuousScheduler in favour of using a local variable to assign self.buffer to before setting the local variable to None

Fixes PY-2140
Fixes #5673

[linear-code]

PY-2140 Investigate potential race conditions in the `teardown` methods of the profilers

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

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New Features ✨

• (ci) Cancel in-progress PR workflows on new commit push by joshuarli in #5994

Bug Fixes 🐛

• (profiler) Stop nulling buffer on teardown by ericapisani in #6075

Internal Changes 🔧

• (pydantic-ai) Remove dead Model.request patch by alexander-alderman-webb in #5956
• Set explicit base-branch for codecov action by ericapisani in #5992

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[github-actions]

Codecov Results 📊

✅ 142 passed | Total: 142 | Pass Rate: 100% | Execution Time: 28.80s

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 14137 uncovered lines.

Files with missing lines (1)

File	Patch %	Lines
continuous_profiler.py	43.41%	⚠️ 176 Missing and 17 partials

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