fix: Reject messages to tasks in terminal states

[kabir]

Per A2A spec CORE-SEND-002, SendMessage must return UnsupportedOperationError when attempting to send messages to tasks in terminal states (completed, canceled, rejected, failed).

Added validation in DefaultRequestHandler.initMessageSend() to check if the existing task is in a terminal state (using TaskState.isFinal()) and throw UnsupportedOperationError before the message reaches the AgentExecutor.

This fixes the issue on all three transports (JSON-RPC, gRPC, HTTP+JSON) since they all use the same DefaultRequestHandler code path.

Fixes #741 🦕

[gemini-code-assist]

Summary of Changes

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This pull request addresses a requirement from the A2A specification (CORE-SEND-002) that prohibits sending messages to tasks that have reached a terminal state. It implements a check within the DefaultRequestHandler to ensure that an UnsupportedOperationError is thrown when such attempts are made, maintaining consistency across different transport protocols.

Highlights

• Spec Compliance: This PR enforces the A2A spec CORE-SEND-002, ensuring that SendMessage returns an UnsupportedOperationError when attempting to send messages to tasks in terminal states.
• Validation Implementation: Validation was added in DefaultRequestHandler.initMessageSend() to check if a task is in a terminal state using TaskState.isFinal().
• Transport Consistency: The fix applies uniformly across all three transports (JSON-RPC, gRPC, HTTP+JSON) via the shared DefaultRequestHandler code path.

Changelog

• DefaultRequestHandler.java
  • Added validation to prevent sending messages to tasks in terminal states, throwing an UnsupportedOperationError if attempted.

Activity

• The PR addresses issue [Bug]: It must not be possible to send a message to a task is in a terminal state #741.
• The PR implements a check in DefaultRequestHandler.initMessageSend() using TaskState.isFinal() to determine if a task is in a terminal state.
• The fix is applied consistently across JSON-RPC, gRPC, and HTTP+JSON transports.

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[kabir]

Duplicate of #746

[gemini-code-assist]

Code Review

This pull request correctly implements the A2A spec requirement to reject messages sent to tasks in terminal states. The change is straightforward and placed in the correct location within DefaultRequestHandler.