fix: UnsupportedOperationError gRPC status should be FAILED_PRECONDITION per spec

[kabir]

Problem

The A2A specification (section 5.4) defines the following gRPC status mapping for UnsupportedOperationError:

A2A Error Type	JSON-RPC Code	gRPC Status	HTTP Status
UnsupportedOperationError	-32004	FAILED_PRECONDITION	400 Bad Request

However, A2AErrorCodes.UNSUPPORTED_OPERATION in the SDK maps to UNIMPLEMENTED:

a2a-java/spec/src/main/java/org/a2aproject/sdk/spec/A2AErrorCodes.java

Line 29 in 643f1f8

UNSUPPORTED_OPERATION(-32004, "UNIMPLEMENTED", 400),

UNSUPPORTED_OPERATION(-32004, "UNIMPLEMENTED", 400),

This diverges from the spec table, which says the gRPC status should be FAILED_PRECONDITION.

Expected behavior

UNSUPPORTED_OPERATION should map to gRPC FAILED_PRECONDITION to match the spec.

Context

Found during review of #1041, which correctly changed streaming-not-supported errors from InvalidRequestError to UnsupportedOperationError. The gRPC status mismatch is a pre-existing issue unrelated to that PR.

[dangzitou]

I reproduced this on current main (643f1f80). I updated only the existing gRPC assertions for paths that emit UnsupportedOperationError and ran:

mvn -pl transport/grpc -am -Dtest=GrpcHandlerTest -Dsurefire.failIfNoSpecifiedTests=false test

All 8 affected paths failed consistently with expected: <FAILED_PRECONDITION> but was: <UNIMPLEMENTED> (42 tests total, 34 unaffected tests passed). I am working on the minimal mapping/test/doc update.