[NOGIL] Fix Producer.close() races with concurrent calls and with itself

[Ojasva Jain (ojasvajain)]

Producer.close() previously raced with concurrent produce()/poll()/ flush()/produce_batch()/transaction calls and with itself when called from multiple threads, both leading to use-after-free/double-free on the underlying rd_kafka_t handle. Adds an active_calls/closing guard (Handle_enter_rk_use/Handle_exit_rk_use) so every method that touches self->rk registers itself before use, and close() drains in-flight calls before tearing down; a CAS on closing ensures only one concurrent close() call performs the actual teardown, with losing callers waiting for it to finish rather than racing it.

Adds tests/parallel/test_producer_close_race.py covering each affected method racing close(), close() racing itself, and close()'s blocking behavior. Uses pytest-forked (POSIX only) so a regression segfault fails only that test.

Integration tests against a real broker are still pending.

What

Checklist

• Contains customer facing changes? Including API/behavior changes
• Did you add sufficient unit test and/or integration test coverage for this PR?
  • If not, please explain why it is not required

References

JIRA:

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Open questions / Follow-ups

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Quality Gate failed

Failed conditions
12.7% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube