shu/task cancel agent logic

[ykeremy]

• task cancelling
• task_canceling API
• task cancel logic in agent

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:rocket:	This description was created by Ellipsis for commit 63502348f392066fa9d96cd259d2d18d99998bf5

Summary:

Introduced task cancellation functionality with a new API endpoint, status transition validation, and handling of invalid transitions.

Key points:

• Introduced task cancellation functionality with a new API endpoint, status transition validation, and handling of invalid transitions.
• Added InvalidTaskStatusTransition and TaskAlreadyCanceled exceptions in skyvern/exceptions.py.
• Updated execute_step, fail_task, and update_task methods in skyvern/forge/agent.py to handle task cancellation and invalid status transitions.
• Added cancel_task endpoint in skyvern/forge/sdk/routes/agent_protocol.py.
• Modified TaskStatus in skyvern/forge/sdk/schemas/tasks.py to include canceled status and updated status transition logic.

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