Detect unclosed cancel scopes when a task exits

[Sakshamm-Goyal]

Summary

Fixes #3329.

A task can enter a CancelScope, return without exiting it, and leave another task able to exit that scope. The existing task-exit checks catch abandoned scope trees and unclosed child nurseries, but not this still-attached scope.

Detect a task exiting with a cancellation status different from its parent nursery's status. Report a scope-stack corruption error, preserving the original exception as context when present, and clean up the unclosed scopes and their registered deadlines. Existing child-nursery recovery is unchanged.

Regression coverage includes nested scopes, cancellation, nursery.start() adoption, guest mode, original exception context and enclosing parent-scope preservation. A release note is included.

Validation

• The issue's real two-task reproduction fails against unchanged main at a3d7cddeacef74c87f667e9f9e8615f2ab7ce894 because no corruption error is raised.
• Current Python 3.10 installed-wheel suite, with slow tests and CI-style isolation/plugin setup: 881 passed, 24 skipped, 2 xfailed.
• Complete core suite: 287 passed, 11 skipped, 1 xfailed.
• Mypy checks over src/trio pass for Darwin, Linux and Windows targets.
• All applicable configured pre-commit hooks pass, including generated-file regeneration, Black, Ruff, spelling and Sphinx lint.
• Python 3.12 full-suite validation has a Jedi __notes__ failure also reproduced on unchanged upstream; Python 3.10's corresponding checks pass. No claim of full platform CI coverage.

Related work

This covers a remaining case after #3307: the exiting task has an unclosed scope but no unclosed child nursery. It does not change the cancelled-task adoption policy discussed in #2896.

AI assistance

This change and its tests were developed and validated with OpenAI Codex assistance.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (a3d7cdd) to head (abc0c31).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3510   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19454        19515   +61     
  Branches         1321         1327    +6     
===============================================
+ Hits            19454        19515   +61     

Files with missing lines	Coverage Δ
src/trio/_core/_run.py	100.00000% <100.00000%> (ø)
src/trio/_core/_tests/test_guest_mode.py	100.00000% <100.00000%> (ø)
src/trio/_core/_tests/test_run.py	100.00000% <100.00000%> (ø)

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