refactor: Moving threadpool configs to the library worker

[gavin-aguiar]

Description

Refactored threadpool lifecycle management from proxy worker to runtime libraries (v1 & v2):

Proxy Worker Changes

• Removed internal threadpool creation/management logic from dispatcher.py
• Removed _sync_call_tp attribute and related methods (_create_sync_call_tp, _stop_sync_call_tp, _get_sync_tp_max_workers)
• Added direct calls to runtime APIs: start_threadpool_executor() during init/reload
• Removed passing threadpool via invocation properties

Runtime Library Additions

• Added threadpool.py modules to both v1 and v2 runtimes
• Implemented centralized threadpool management:
• start_threadpool_executor() - creates/replaces executor with PYTHON_THREADPOOL_THREAD_COUNT validation
• stop_threadpool_executor() - graceful shutdown
• get_threadpool_executor() - thread-safe accessor
• Updated handle_event.py in both runtimes to fetch executor internally
• Added public API exports in runtime init.py files

Testing

• Added comprehensive unit tests for threadpool functionality (v1 & v2)
• Added dispatcher tests validating threadpool delegation behavior
• Fixed flake8 style issues across modified files
• Fixed v2 runtime flake8 test discovery (repo root resolution)

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Pull Request Checklist

Host-Worker Contract

• Does this PR impact the host-worker contract (e.g., gRPC messages, shared interfaces)?
  • If yes, have the changes been applied to:
    • azure_functions_worker (Python <= 3.12)
    • proxy_worker (Python >= 3.13)
  • If no, please explain why:

Worker Execution Logic

• Does this PR affect worker execution logic (e.g., function invocation, bindings, lifecycle)?
  If yes, please answer the following:

Python Version Coverage

• Does this change apply to both Python <=3.12 and 3.13+?
• If yes, have the changes been made to:
  • azure_functions_worker (Python <= 3.12)
  • runtimes/v1 / runtimes/v2 (Python >= 3.13)
• If no, please explain why:

Programming Model Compatibility (for Python 3.13+)

• Does this change apply to both:
  • V1 programming model (runtimes/v1)?
  • V2 programming model (runtimes/v2)?
• Explanation (if limited to one model):