fix: reject gRPC message without a messageId (a2aproject#569)

Failure is reported in the TCK by the
tests/optional/features/test_invalid_business_logic.py::test_missing_required_message_fields
test.

Signed-off-by: Jeff Mesnil <[email protected]>

Author: jmesnil

spec-grpc/src/main/java/io/a2a/grpc/mapper/A2ACommonFieldMapper.java

@@ -14,6 +14,8 @@
 import org.mapstruct.Mapper;
 import org.mapstruct.Named;
 
+import io.a2a.spec.InvalidParamsError;
+
 /**
  * Common field mapping utilities shared across all mappers.
  * <p>
@@ -46,6 +48,24 @@ default String emptyToNull(String value) {
         return (value == null || value.isEmpty()) ? null : value;
     }
 
+    /**
+     * Validates that a required string field is not null or empty.
+     * <p>
+     * Throws an exception if the protobuf string is null or empty.
+     * Use this with {@code @Mapping(qualifiedByName = "requireNonEmpty")}.
+     *
+     * @param value the protobuf string value
+     * @return the value if not null/empty
+     * @throws IllegalArgumentException if value is null or empty
+     */
+    @Named("requireNonEmpty")
+    default String requireNonEmpty(String value) {
+        if (value == null || value.isEmpty()) {
+            throw new InvalidParamsError("Required field cannot be null or empty");
+        }
+        return value;
+    }
+
     /**
      * Converts null strings to empty strings for protobuf.
      * <p>

spec-grpc/src/main/java/io/a2a/grpc/mapper/MessageMapper.java

@@ -36,7 +36,7 @@ public interface MessageMapper {
      * Uses Builder pattern explicitly configured via @BeanMapping.
      */
     @BeanMapping(builder = @Builder(buildMethod = "build"))
-    @Mapping(target = "messageId", source = "messageId", qualifiedByName = "emptyToNull")
+    @Mapping(target = "messageId", source = "messageId", qualifiedByName = "requireNonEmpty")
     @Mapping(target = "contextId", source = "contextId", qualifiedByName = "emptyToNull")
     @Mapping(target = "taskId", source = "taskId", qualifiedByName = "emptyToNull")
     @Mapping(target = "metadata", source = "metadata", qualifiedByName = "metadataFromProto")