feat(a2a-bridge): add gRPC transport

[zeroasterisk]

Summary

Add gRPC transport to the A2A bridge, implementing all 11 A2A protocol RPCs. Thin adapter layer — delegates to the same Bridge methods the JSON-RPC server uses, zero duplicated business logic.

All RPCs: SendMessage, SendStreamingMessage, GetTask, ListTasks, CancelTask, SubscribeToTask, Create/Get/List/DeleteTaskPushNotificationConfig, GetExtendedAgentCard.

Config: set bridge.grpc_listen_address: ":9443" to enable.

Design doc: .design/a2a-grpc-transport.md

Test plan

• 22 new tests covering type translation, RPC handling, error mapping
• All existing tests pass

Note: if #315 (follow-up/SendMessage signature change) merges first, trivial 1-line fix needed (add empty taskID param).

[gemini-code-assist]

Code Review

This pull request implements gRPC transport support for the A2A bridge, introducing a gRPC server adapter, proto definitions, and corresponding tests. The review feedback highlights several issues in the gRPC server implementation: a parsing bug in resolveProject due to fmt.Sscanf capturing too much of the tenant string, potential data loss in internalPartToProto where structured p.Data is ignored, and missing validation for the required id field in both GetTaskPushNotificationConfig and DeleteTaskPushNotificationConfig handlers.

[gemini-code-assist]

The current implementation of resolveProject uses fmt.Sscanf with %s to parse the tenant string. In Go, %s matches any sequence of non-space characters, meaning it will consume the entire rest of the string (including / characters). As a result, parsing "projects/{projectSlug}/agents/{agentSlug}" will fail because p will capture "my-project/agents/my-agent", causing the subsequent Sscanf to fail.

Using strings.HasPrefix and strings.Split is much more robust, readable, and correct. Please make sure to import "strings" in this file.

func (s *GRPCServer) resolveProject(tenant string) (projectSlug, agentSlug string, err error) {
	if tenant == "" {
		if len(s.config.Projects) > 0 {
			return s.config.Projects[0].Slug, "", fmt.Errorf("tenant is required to identify the agent")
		}
		return "", "", fmt.Errorf("tenant is required")
	}

	// Handle "projects/{projectSlug}/agents/{agentSlug}" format
	if strings.HasPrefix(tenant, "projects/") {
		trimmed := strings.TrimPrefix(tenant, "projects/")
		parts := strings.Split(trimmed, "/agents/")
		if len(parts) == 2 && parts[0] != "" && parts[1] != "" {
			return parts[0], parts[1], nil
		}
		return "", "", fmt.Errorf("invalid tenant format: %s", tenant)
	}

	// Handle simple "projectSlug/agentSlug" format
	parts := strings.Split(tenant, "/")
	if len(parts) == 2 && parts[0] != "" && parts[1] != "" {
		return parts[0], parts[1], nil
	}

	return "", "", fmt.Errorf("invalid tenant format: %s", tenant)
}

[gemini-code-assist]

The internalPartToProto function currently ignores p.Data (structured data). If the bridge returns a task with artifacts containing structured data, this data will be lost when translating back to protobuf, and the client will receive an empty part. We should convert p.Data back to pb.Part_Data using structpb.NewValue(p.Data). Please make sure to import "google.golang.org/protobuf/types/known/structpb" in this file.

func internalPartToProto(p Part) *pb.Part {
	pbPart := &pb.Part{
		MediaType: p.MediaType,
	}
	switch {
	case p.Text != "":
		pbPart.Content = &pb.Part_Text{Text: p.Text}
	case p.URL != "":
		pbPart.Content = &pb.Part_Url{Url: p.URL}
	case p.Data != nil:
		if val, err := structpb.NewValue(p.Data); err == nil {
			pbPart.Content = &pb.Part_Data{Data: val}
		}
	}
	return pbPart
}

[gemini-code-assist]

The id field is marked as REQUIRED in the GetTaskPushNotificationConfigRequest protobuf message, but it is not validated here. If a client sends an empty ID, it will query the database and then return NotFound instead of InvalidArgument. We should validate that id is not empty.

func (s *GRPCServer) GetTaskPushNotificationConfig(ctx context.Context, req *pb.GetTaskPushNotificationConfigRequest) (*pb.TaskPushNotificationConfig, error) {
	if req.GetTaskId() == "" {
		return nil, grpcstatus.Errorf(codes.InvalidArgument, "task_id is required")
	}
	if req.GetId() == "" {
		return nil, grpcstatus.Errorf(codes.InvalidArgument, "id is required")
	}

[gemini-code-assist]

The id field is marked as REQUIRED in the DeleteTaskPushNotificationConfigRequest protobuf message, but it is not validated here. We should validate that id is not empty to prevent calling the underlying bridge method with an empty ID.

func (s *GRPCServer) DeleteTaskPushNotificationConfig(ctx context.Context, req *pb.DeleteTaskPushNotificationConfigRequest) (*emptypb.Empty, error) {
	if req.GetTaskId() == "" {
		return nil, grpcstatus.Errorf(codes.InvalidArgument, "task_id is required")
	}
	if req.GetId() == "" {
		return nil, grpcstatus.Errorf(codes.InvalidArgument, "id is required")
	}

[zeroasterisk]

Round 3 review: 4 more bugs found and fixed. This is the PR that keeps producing findings — the gRPC type translation layer has many edge cases. All fixed, 3 clean cycles after. Rebased on upstream/main, all tests pass.

[zeroasterisk]

Closing in favor of #362 (a2a-go SDK migration) which replaces the hand-rolled gRPC with the official SDK's transport layer, providing JSON-RPC + gRPC + REST from a single AgentExecutor implementation.