fix task workflow cancel issue

[wintonzheng]

[!IMPORTANT] Refactor task and workflow cleanup processes, introduce clean_up_task and clean_up_workflow methods, and switch to httpx for async requests.

• Behavior:
  • Introduces clean_up_task in agent.py to replace send_task_response, handling task cleanup and webhook calls.
  • Modifies execute_step in agent.py to use clean_up_task for task cancellation and failure scenarios.
  • Updates execute_workflow in service.py to use clean_up_workflow for workflow cleanup.
• Exceptions:
  • Updates MissingBrowserState in exceptions.py to include workflow_run_id.
• Browser Management:
  • Refactors get_or_create_for_task and get_or_create_for_workflow_run in browser_manager.py to improve browser state handling.
  • Adds get_for_task and get_for_workflow_run methods in browser_manager.py for retrieving existing browser states.
• Misc:
  • Replaces requests with httpx for async HTTP requests in agent.py and service.py.
  • Updates logging and error handling in agent.py and service.py for better clarity and debugging.

^{This description was created by for 35382512734ad347ec00daa3a948ee3938990558. It will automatically update as commits are pushed.}