fix: Include google.rpc.ErrorInfo in gRPC error status details

[kabir]

Per specification requirement GRPC-ERR-001 (section 10.6), A2A-specific errors must include a google.rpc.ErrorInfo message in the status.details array with:

• reason: A2A error type in UPPER_SNAKE_CASE without "Error" suffix
• domain: "a2a-protocol.org"
• metadata: Optional map of additional error context

Modified GrpcHandler.handleError() to:

1. Create google.rpc.ErrorInfo with appropriate reason and domain
2. Build google.rpc.Status containing the ErrorInfo in details
3. Include Status in grpc-status-details-bin trailing metadata
4. Rename ExtendedCardNotConfiguredError to ExtendedAgentCardNotConfiguredError. This wasn't part of the original work done in this PR, but was picked up by the Gemini review, so I included it

This ensures gRPC error responses comply with the A2A specification by providing structured error information that clients can programmatically inspect and handle.

Fixes #731

[gemini-code-assist]

Summary of Changes

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This pull request enhances the gRPC error handling mechanism to align with the A2A specification. By embedding structured error details, specifically google.rpc.ErrorInfo, into gRPC responses, clients can now programmatically parse and react to specific error types, improving the robustness and interoperability of the A2A protocol.

Highlights

• gRPC Error Specification Compliance: Implemented GRPC-ERR-001 (section 10.6) specification requirement to include google.rpc.ErrorInfo in gRPC error status details for A2A-specific errors.
• Structured Error Information: Modified GrpcHandler.handleError() to construct and embed google.rpc.ErrorInfo messages within the status.details array of google.rpc.Status objects.
• Metadata Inclusion: Ensured the google.rpc.Status object, containing the ErrorInfo, is included in the grpc-status-details-bin trailing metadata of gRPC error responses.
• Error Reason Mapping: Mapped various A2AError types to specific UPPER_SNAKE_CASE reason strings (e.g., INVALID_REQUEST, METHOD_NOT_FOUND) for the ErrorInfo.

Changelog

• transport/grpc/src/main/java/io/a2a/transport/grpc/handler/GrpcHandler.java
  • Introduced an errorReason variable to categorize different A2AError types.
  • Assigned specific UPPER_SNAKE_CASE reason strings to errorReason based on the type of A2AError.
  • Constructed a com.google.rpc.ErrorInfo object using the determined errorReason and a fixed domain of "a2a-protocol.org".
  • Built a com.google.rpc.Status object, incorporating the original gRPC status code and description, and adding the ErrorInfo to its details.
  • Created io.grpc.Metadata trailers and populated them with the com.google.rpc.Status object under the grpc-status-details-bin key.
  • Modified the responseObserver.onError call to pass these newly constructed trailers, ensuring structured error details are sent to the client.

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[gemini-code-assist]

Code Review

This pull request correctly implements the specification requirement to include google.rpc.ErrorInfo in gRPC error status details for A2A-specific errors. The changes in GrpcHandler.handleError properly construct the ErrorInfo and attach it to the response metadata. I've added a couple of suggestions to improve code maintainability and performance within the modified handleError method.

[jmesnil]

@kabir I have a look at the v1.0.0 spec and they changed that part in a2aproject/A2A#1600. We will have to revisit it, #731 is now out of date

[ehsavoie]

Shouldn't we handle this at the A2AError level ?