Enable free-threaded Python and improve concurrency in CEL Python.

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Enable free-threaded Python and improve concurrency in CEL Python.

• Declare free-threading compatibility on pybind11 modules (mod_gil_not_used) and BUILD targets (freethreading_compatible).
• Introduce FreeThreadingMutex, a zero-cost abstraction that compiles away in GIL-enabled builds and uses absl::Mutex in free-threaded mode (Py_GIL_DISABLED).
• Protect lazy cel_program_ compilation in PyCelExpression with FreeThreadingMutex.
• Protect Python object caching and lazy element resolution in PyCelValue and list/map accessors with FreeThreadingMutex.
• Protect PyMessageFactory::message_classes_ with FreeThreadingMutex.
• Extract BuildCompiler and BuildRuntime to construct compiler/runtime objects without holding mutex locks, using double-checked locking in GetCompiler and GetRuntime.
• Eagerly resolve variable_types_ during PyCelEnvInternal initialization, making GetVariableType lock-free and thread-safe.
• Release GIL during CPU-bound policy compilation and YAML parsing.
• Avoid unconditional gil_scoped_acquire across destructors and error conversions when GIL state is already held (PyGILState_Check).
• Initialize pybind11 internals at module import time to prevent DSO internals races in multi-threaded environments.
• Prevent reference leaks in custom function invocation by releasing arguments and temporary results.
• Add test_freethreaded.sh script and concurrency tests in cel_parallel_test.py and cel_test.py.

Performance Benchmarks (cel_parallel_test, -c opt)

Benchmark Stage / Test Case	Standard Mode (GIL)	Free-Threaded Mode (No GIL)	Speedup / Impact
Multi-Threaded Compilation (1,000 items, 8 threads)	75.74 ms	23.14 ms	3.27x faster (69.5% faster)
Sequential Compilation (1,000 items)	61.77 ms	63.28 ms	0.98x
Multi-Threaded Evaluation (10,000 items, 8 threads)	234.79 ms	138.88 ms	1.69x faster (40.9% faster)
Sequential Evaluation (10,000 items)	90.68 ms	112.74 ms	0.80x