Fix Producer.poll wakeable early return

[Jeremy Gu (wgu9)]

Fixes #2215.

Summary

• return from wakeable Producer.poll() once rd_kafka_poll() serves producer events
• tighten the existing wakeable producer integration test so a full-timeout wait after delivery is caught
• add a changelog entry

Validation

• git diff --check
• .venv/bin/python -m py_compile tests/integration/producer/test_producer_wakeable_poll_flush.py
• .venv/bin/black --check tests/integration/producer/test_producer_wakeable_poll_flush.py
• .venv/bin/isort --check-only tests/integration/producer/test_producer_wakeable_poll_flush.py
• .venv/bin/flake8 tests/integration/producer/test_producer_wakeable_poll_flush.py
• built editable install against librdkafka v2.15.0-RC1
• DYLD_LIBRARY_PATH=/private/tmp/librdkafka-2.15.0-RC1-install/lib .venv/bin/python -c "import confluent_kafka; print(confluent_kafka.version()); print(confluent_kafka.libversion())"
• DYLD_LIBRARY_PATH=/private/tmp/librdkafka-2.15.0-RC1-install/lib .venv/bin/pytest -q tests/test_Wakeable.py::test_producer_wakeable_poll_interruptibility_and_messages tests/test_Wakeable.py::test_producer_wakeable_poll_edge_cases

I also attempted tests/integration/producer/test_producer_wakeable_poll_flush.py::test_poll_message_delivery_with_wakeable_pattern; it reached trivup cluster setup but could not complete locally because the Docker daemon was not running for confluentinc/cp-schema-registry:7.6.0.

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