fix: omit optional fields with default values from client JSON serialization

The JSON-RPC client serialization used alwaysPrintFieldsWithNoPresence()
which caused proto3 implicit-presence fields (taskId, contextId, tenant, etc.)
to be serialized as empty strings instead of being omitted.

Removed alwaysPrintFieldsWithNoPresence() from JsonFormat.printer() in
JSONRPCUtils.toJsonRPCRequest() (client requests only), aligning with the
REST client transport which already omitted it.

This fixes #770

Author: jmesnil

client/transport/jsonrpc/src/test/java/io/a2a/client/transport/jsonrpc/JsonMessages.java

@@ -93,10 +93,7 @@ public class JsonMessages {
                     {
                       "text":"tell me a joke"
                     }
-                  ],
-                  "metadata":{
-                    
-                  }
+                  ]
                 }
               }
             }""";
@@ -143,18 +140,12 @@ public class JsonMessages {
                 {
                   "text":"tell me a joke"
                 }
-              ],
-              "metadata":{
-              }
+              ]
             },
             "configuration":{
               "acceptedOutputModes":[
                 "text"
-              ],
-              "returnImmediately":false
-            },
-            "metadata":{
-
+              ]
             }
           }
         }""";
@@ -192,19 +183,12 @@ public class JsonMessages {
                     {
                       "text":"tell me a joke"
                     }
-                  ],
-                  "metadata":{
-                    
-                  }
+                  ]
                 },
                 "configuration":{
                   "acceptedOutputModes":[
                     "text"
-                  ],
-                  "returnImmediately":false
-                },
-                "metadata":{
-                  
+                  ]
                 }
               }
             }""";
@@ -376,19 +360,12 @@ public class JsonMessages {
                        "url":"file:///path/to/image.jpg",
                        "mediaType":"image/jpeg"
                      }
-                   ],
-                   "metadata":{
-                     
-                   }
+                   ]
                  },
                  "configuration":{
                    "acceptedOutputModes":[
                      "text"
-                   ],
-                   "returnImmediately":false
-                 },
-                 "metadata":{
-                   
+                   ]
                  }
                }
              }""";
@@ -443,19 +420,12 @@ public class JsonMessages {
                         "timestamp":"2024-01-15T10:30:00Z"
                       }
                     }
-                  ],
-                  "metadata":{
-                    
-                  }
+                  ]
                 },
                 "configuration":{
                   "acceptedOutputModes":[
                     "text"
-                  ],
-                  "returnImmediately":false
-                },
-                "metadata":{
-                  
+                  ]
                 }
               }
             }""";
@@ -514,19 +484,12 @@ public class JsonMessages {
                         "labels":["Q1", "Q2", "Q3", "Q4"]
                       }
                     }
-                  ],
-                  "metadata":{
-                    
-                  }
+                  ]
                 },
                 "configuration":{
                   "acceptedOutputModes":[
                     "text"
-                  ],
-                  "returnImmediately":false
-                },
-                "metadata":{
-                  
+                  ]
                 }
               }
             }""";

spec-grpc/src/main/java/io/a2a/grpc/utils/JSONRPCUtils.java

@@ -171,6 +171,9 @@ public class JSONRPCUtils {
     private static final Pattern EXTRACT_WRONG_TYPE = Pattern.compile("Expected (.*) but found \".*\"");
     static final String ERROR_MESSAGE = "Invalid request content: %s. Please verify the request matches the expected schema for this method.";
 
+    private static final JsonFormat.Printer REQUEST_PRINTER = JsonFormat.printer().omittingInsignificantWhitespace();
+    private static final JsonFormat.Printer RESPONSE_PRINTER = JsonFormat.printer().alwaysPrintFieldsWithNoPresence().omittingInsignificantWhitespace();
+
     public static A2ARequest<?> parseRequestBody(String body, @Nullable String tenant) throws JsonMappingException, JsonProcessingException {
         JsonElement jelement = JsonParser.parseString(body);
         JsonObject jsonRpc = jelement.getAsJsonObject();
@@ -556,7 +559,7 @@ public static String toJsonRPCRequest(@Nullable String requestId, String method,
                 output.name("method").value(method);
             }
             if (payload != null) {
-                String resultValue = JsonFormat.printer().alwaysPrintFieldsWithNoPresence().omittingInsignificantWhitespace().print(payload);
+                String resultValue = REQUEST_PRINTER.print(payload);
                 output.name("params").jsonValue(resultValue);
             }
             output.endObject();
@@ -579,7 +582,7 @@ public static String toJsonRPCResultResponse(Object requestId, com.google.protob
                     output.name("id").value(number.longValue());
                 }
             }
-            String resultValue = JsonFormat.printer().alwaysPrintFieldsWithNoPresence().omittingInsignificantWhitespace().print(builder);
+            String resultValue = RESPONSE_PRINTER.print(builder);
             output.name("result").jsonValue(resultValue);
             output.endObject();
             return result.toString();