feat: include Function in FunctionResultSubContent

PiperOrigin-RevId: 1005139669

Author: IvanLH

src/main/java/com/google/genai/gaos/models/interactions/Function.java

@@ -38,7 +38,7 @@
  * 
  * <p>A tool that can be used by the model.
  */
-public class Function implements Tool, AgentTool {
+public class Function implements FunctionResultSubcontent, Tool, AgentTool {
     /**
      * A description of the function.
      */

src/main/java/com/google/genai/gaos/models/interactions/FunctionResultSubcontentTypeIdResolver.java

@@ -37,6 +37,7 @@ public FunctionResultSubcontentTypeIdResolver() {
     private void initializeTypeMap() {
         registerType("audio", AudioContent.class);
         registerType("document", DocumentContent.class);
+        registerType("function", Function.class);
         registerType("image", ImageContent.class);
         registerType("text", TextContent.class);
         registerType("video", VideoContent.class);