Propagate MemoryError from pool callbacks

[sahilmathur254]

Summary

• Propagate MemoryError raised by pool callbacks instead of logging and
  swallowing it.
• Add regression coverage through the ApplyResult._set() callback path.

On Python 3, MemoryError is an Exception, so the generic callback handler
currently catches it. Celery's request callback expects this exception to escape,
and swallowing it can leave an acks_late message unacknowledged. The explicit
MemoryError branch restores propagation without changing how other callback
exceptions or callbacks_propagate are handled.

Related to celery/celery#10462. A Celery regression around Request.on_failure
remains a follow-up after the billiard behavior is available.

Validation

All tests were run in an isolated Python 3.12 Docker container.

• Before the fix, the new regression failed with DID NOT RAISE MemoryError and
  logged the callback exception.
• python -m pytest t/unit/test_pool.py -q: 6 passed.
• python -m pytest t/unit -q: 39 passed, 51 skipped.
• python -m compileall -q billiard t/unit: passed.
• git diff --check: passed.

AI disclosure

This PR includes AI-assisted code understanding to speed up the process; the
implementation and tests were updated and reviewed manually with AI assistance.

[Copilot]

Pull request overview

This PR updates billiard.pool.ApplyResult callback execution so that MemoryError raised inside pool callbacks is propagated instead of being logged and swallowed, matching Celery’s expectation on Python 3 and avoiding situations where acks_late tasks can remain unacknowledged.

Changes:

• Update ApplyResult.safe_apply_callback() to explicitly re-raise MemoryError (while preserving existing callbacks_propagate behavior for other exception types).
• Add a unit regression test that exercises the ApplyResult._set() → callback path and asserts MemoryError propagation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
billiard/pool.py	Ensures MemoryError from ApplyResult callbacks escapes instead of being logged/swallowed.
t/unit/test_pool.py	Adds regression coverage proving MemoryError propagates through ApplyResult._set() callback execution.

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