fix(grpc): align unsupported operation status

[dangzitou]

Description

Align UnsupportedOperationError with the A2A gRPC specification by mapping it to FAILED_PRECONDITION instead of UNIMPLEMENTED. Update the existing server and client mapping assertions, plus the handler mapping documentation.

• Follow the CONTRIBUTING Guide.
• Use a Conventional Commits PR title.
• Ensure the affected tests pass.
• Appropriate documentation was updated.

Reproduction

On current main, changing only the eight existing GrpcHandlerTest expectations for paths that emit UnsupportedOperationError produced 8 consistent failures out of 42 tests:

expected: <FAILED_PRECONDITION> but was: <UNIMPLEMENTED>

Verification

Run with OpenJDK 17:

mvn -DskipTests install -pl transport/grpc,client/transport/grpc -am
mvn -pl transport/grpc test
mvn -pl client/transport/grpc test

Results:

• transport/grpc: 42 tests passed
• client/transport/grpc: 9 tests passed
• all 12 prerequisite/affected reactor modules compiled and installed successfully

A root mvn clean install was also attempted, but the unrelated client/transport/jsonrpc tests could not bind/use their hard-coded local endpoint because 127.0.0.1:4001 was already occupied by another local application. The two changed gRPC modules were fully tested separately above.

This fixes #1051