Fix gRPC status details trailer handling

[hutiefang76]

Fixes gh-4224

This preserves grpc-status-details-bin when GRPCResponseHeadersFilter moves a trailers-only gRPC response status into Reactor Netty trailer headers.

Previously the immediate error path copied only grpc-status and grpc-message. grpc-status-details-bin is also gRPC trailer metadata, so clients lost the rich error details for trailers-only responses.

Changes:

• copy grpc-status-details-bin into the trailer headers when present
• add a focused unit test that captures the Reactor Netty trailerHeaders consumer and verifies grpc-status, grpc-message, and grpc-status-details-bin

Verification:

• RED: GRPCResponseHeadersFilterTests.shouldIncludeGrpcStatusDetailsBinTrailerIfGRPC failed before the fix because grpc-status-details-bin was null
• GREEN: JAVA_HOME=/Users/hutiefang/Library/Java/JavaVirtualMachines/ms-17.0.18/Contents/Home ./mvnw -pl spring-cloud-gateway-server-webflux -Dtest=GRPCResponseHeadersFilterTests test
• Related tests: same command with -Dtest=GRPCRequestHeadersFilterTest,GRPCResponseHeadersFilterTests passed 3 tests
• Format: JAVA_HOME=/Users/hutiefang/Library/Java/JavaVirtualMachines/ms-17.0.18/Contents/Home ./mvnw -pl spring-cloud-gateway-server-webflux spring-javaformat:validate
• git diff --check