[NOGIL] Declare cimpl free-threaded on CPython 3.14t

[Honglei (honglei)]

Relates to #2092

What

Declare the CPython 3.14t cimpl extension as free-threaded and protect the
lazy ShareConsumer Messages type cache when the GIL is disabled.

This is based on dev_thread_free_support, which already provides the 3.14t
build and test dependency preparation.

Changes

• Call PyUnstable_Module_SetGIL(m, Py_MOD_GIL_NOT_USED) for free-threaded
  builds.
• Protect the process-lifetime Messages type initialization with PyMutex
  on free-threaded builds.
• Add isolated free-threading tests for independent and shared Producer use.
• Add an integration test using the existing kafka_cluster fixture to cover
  shared AdminClient, Producer, and Consumer operations.
• Run the strict free-threading tests in a separate process before the full
  regression suite. The full suite still reports warnings when optional
  third-party extensions, such as fastavro, re-enable the GIL.

Validation

Validated on CPython 3.14.6 free-threading at /opt/python314t on 10.2:

• Formatting, shell syntax, and diff checks passed.
• Isolated no-GIL tests: 3 passed, 1 skipped without a broker.
• Real-broker no-GIL tests: 4 passed against 127.0.0.1:19092 before wiring
  the test into the project integration fixture.
• Final kafka_cluster integration test against the same broker:
  1 passed in 1.42s with PYTHON_GIL=0.
• Core regression tests: 108 passed.
• pip check: no broken requirements.
• Earlier 60-second real-broker soak: 99 rounds, approximately 198,000
  messages, zero failures, and no leftover temporary topics.

The integration test is run in a separate PYTHON_GIL=0 process because the
full test process imports optional extensions such as fastavro that can
re-enable the GIL. The existing source verification job already prepares the
Kafka artifact used by the kafka_cluster fixture.

Scope

This change does not claim concurrent close while another thread is still
calling poll or consume. Callers must stop worker threads before closing a
shared client.

[confluent-cla-assistant]

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