change tool execution response key to result

Author: nikoncode

core/src/main/java/com/google/adk/tools/mcp/AbstractMcpTool.java

@@ -161,6 +161,6 @@ protected static Map<String, Object> wrapCallResult(
         resultMaps.add(ImmutableMap.of("text", textOutput));
       }
     }
-    return ImmutableMap.of("text_output", resultMaps);
+    return ImmutableMap.of("result", resultMaps);
   }
 }