Fix frozen TaskFailedEvent mutation in workforce failure handling

[LuoPengcheng12138]

Description

This PR fixes a runtime error caused by mutating a Pydantic frozen event model. In Workforce._handle_failed_task, we created a TaskFailedEvent and then attempted to set worker_id and failure_count afterward. CAMEL’s event models are ConfigDict(frozen=True, extra='forbid'), so post‑creation assignment raises ValidationError: Instance is frozen.

The fix passes worker_id directly in the constructor and moves failure_count into metadata, which is an allowed field. This keeps the event immutable and avoids the validation error during failure handling.

[error] BACKEND: 2026-02-02 22:09:29,434 - camel.camel.societies.workforce.workforce - ERROR - Error handling failed task 1770041310698-1927.1: 1 validation error for TaskFailedEvent
worker_id
  Instance is frozen [type=frozen_instance, input_value='3796b026-9380-46c9-bc42-86ad673fdee2', input_type=str]
    For further information visit https://errors.pydantic.dev/2.12/v/frozen_instance
Traceback (most recent call last):
  File "/Users/Luopc/.eigent/venvs/backend-0.0.82/lib/python3.10/site-packages/camel/societies/workforce/workforce.py", line 5473, in _listen_to_channel
    halt = await self._handle_failed_task(returned_task)
  File "/Users/Luopc/Documents/Development/MyLLMs/eigent/backend/app/utils/workforce.py", line 698, in _handle_failed_task
    event.worker_id = task.assigned_worker_id
  File "/Users/Luopc/.eigent/venvs/backend-0.0.82/lib/python3.10/site-packages/pydantic/main.py", line 1032, in __setattr__
    elif (setattr_handler := self._setattr_handler(name, value)) is not None:
  File "/Users/Luopc/.eigent/venvs/backend-0.0.82/lib/python3.10/site-packages/pydantic/main.py", line 1068, in _setattr_handler
    _check_frozen(cls, name, value)
  File "/Users/Luopc/.eigent/venvs/backend-0.0.82/lib/python3.10/site-packages/pydantic/main.py", line 89, in _check_frozen
    raise ValidationError.from_exception_data(
pydantic_core._pydantic_core.ValidationError: 1 validation error for TaskFailedEvent
worker_id
  Instance is frozen [type=frozen_instance, input_value='3796b026-9380-46c9-bc42-86ad673fdee2', input_type=str]
    For further information visit https://errors.pydantic.dev/2.12/v/frozen_instance

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other