Fix Cancelled.__context__ leaking from other tasks

[Fridayai700]

Summary

Fixes #2649

When task 1 calls cancel() on a scope while handling an exception, the Cancelled exception delivered to task 2 (sleeping in that scope) incorrectly inherits task 1's exception as __context__. This is because capture(raise_cancel) is called within task 1's exception handling context, so Python automatically sets __context__ on the newly-raised Cancelled.

Before

async with trio.open_nursery() as nursery:
    cancel_scope = trio.CancelScope()
    nursery.start_soon(task2, cancel_scope)
    await trio.testing.wait_all_tasks_blocked()

    try:
        raise RuntimeError("task1 error")
    except RuntimeError:
        cancel_scope.cancel()
    # task2's Cancelled.__context__ == RuntimeError("task1 error")  ← wrong!

After

    # task2's Cancelled.__context__ is None  ← correct

Fix

Clear __context__ on the captured error in _attempt_abort before rescheduling the cancelled task. This matches the behavior of other abort pathways (deferred cancellation like IOCP/io_uring) where the Cancelled exception naturally gets __context__ = None.

The local variable is explicitly deleted to avoid creating cyclic garbage (verified by the existing test_simple_cancel_scope_usage_doesnt_create_cyclic_garbage test).

Test

Added test_cancelled_context_does_not_leak_from_other_task which:

1. Starts task 2 sleeping in a cancel scope
2. Has task 1 cancel the scope while handling a RuntimeError
3. Verifies task 2's Cancelled exception has __context__ is None

The test fails without the fix (Cancelled.context == RuntimeError) and passes with it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (2fd138e) to head (9fcf542).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3395   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19424        19447   +23     
  Branches         1318         1318           
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+ Hits            19424        19447   +23     

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

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[A5rocks]

We have a cleaner way to do this, blocked on some other PRs.