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

@@ -556,7 +556,7 @@ 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 = JsonFormat.printer().omittingInsignificantWhitespace().print(payload);
                 output.name("params").jsonValue(resultValue);
             }
             output.endObject();