feat: send task as a first subscribe event

[ishymko]

Following

The operation MUST return a Task object as the first event in the stream

introduced in 1.0: https://a2a-protocol.org/latest/specification/#316-subscribe-to-task

It also correctly uses UnsupportedOperationError as per the "Errors" section from the documentation linked above.

Fixes #675

[gemini-code-assist]

Summary of Changes

Hello @ishymko, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request brings the subscribe_to_task operation into full compliance with version 1.0 of the A2A protocol specification. It ensures that consumers of the task stream receive the Task object as the very first event, providing immediate context. Additionally, it refines the error reporting for attempts to subscribe to tasks that are already in a completed state, aligning the error type with the specified UnsupportedOperationError.

Highlights

• Protocol Compliance: Implemented the A2A protocol specification 1.0 requirement to send the Task object as the first event in the subscribe_to_task stream.
• Error Handling Update: Updated error handling for subscribe_to_task when a task is in a terminal state, now raising UnsupportedOperationError instead of InvalidParamsError as per the specification.
• Testing: Added and modified unit tests to ensure compliance with the new subscribe_to_task behavior and error handling.

Changelog

• src/a2a/server/request_handlers/default_request_handler.py
  • The on_subscribe_to_task asynchronous generator now yields the Task object as its first event.
  • The error raised when subscribing to a task in a terminal state was changed from InvalidParamsError to UnsupportedOperationError.
• tests/server/request_handlers/test_default_request_handler.py
  • A new assertion was added to test_on_subscribe_to_task_resubscribe to confirm the first event received is the Task object.
  • The test_on_subscribe_to_task_in_terminal_state test was updated to expect UnsupportedOperationError.

Activity

• Addressed and fixed issue [Task]: 1.0: Send current task state as a first event on "Subscribe to Task" #675.

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[gemini-code-assist]

Code Review

This pull request correctly implements changes to align with the A2A protocol specification. It ensures that subscribing to a task stream now correctly yields the task object as the first event and raises an UnsupportedOperationError when subscribing to a task in a terminal state. The tests have been updated accordingly to verify this new behavior. I have one minor suggestion to improve code consistency in the test suite.