Fix/od initialization threadsafe

[ajjimeno]

Make OD model initialization threadsafe.

---

Note

Make get_model thread-safe with a lock, add a multithreaded test, and bump version to 1.0.9 with changelog update.

• Models:
  • Thread-safe initialization in unstructured_inference/models/base.py by adding models_lock and wrapping get_model initialization path with a lock and double-check cache.
• Tests:
  • Add test_get_model_threaded in test_unstructured_inference/models/test_model.py to validate concurrent get_model calls.
• Versioning:
  • Bump __version__ to 1.0.9.
  • Update CHANGELOG.md with 1.0.9 entry about thread-safe OD model loading.

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