fix: Returns dict as result from McpTool

The `BaseTool` expects the run_async to return a json-serializable object. By model_dump the McpTool result explicitly can allow what ADK runtime sees is identical to what is persisted in the session event list.

Before the change, runtime sees CallToolResult instance and Session persists its serialized dict.

https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/types.py#L916-L922

PiperOrigin-RevId: 822465432

src/google/adk/tools/mcp_tool/mcp_tool.py

@@ -80,7 +80,7 @@ class McpTool(BaseAuthenticatedTool):
           Callable[[ReadonlyContext], Dict[str, str]]
       ] = None,
   ):
-    """Initializes an MCPTool.
+    """Initializes an McpTool.
 
     This tool wraps an MCP Tool interface and uses a session manager to
     communicate with the MCP server.
@@ -186,7 +186,7 @@ class McpTool(BaseAuthenticatedTool):
   @override
   async def _run_async_impl(
       self, *, args, tool_context: ToolContext, credential: AuthCredential
-  ):
+  ) -> Dict[str, Any]:
     """Runs the tool asynchronously.
 
     Args:
@@ -217,7 +217,7 @@ class McpTool(BaseAuthenticatedTool):
     )
 
     response = await session.call_tool(self._mcp_tool.name, arguments=args)
-    return response
+    return response.model_dump(exclude_none=True, mode="json")
 
   async def _get_headers(
       self, tool_context: ToolContext, credential: AuthCredential
@@ -282,7 +282,7 @@ class McpTool(BaseAuthenticatedTool):
             != APIKeyIn.header
         ):
           error_msg = (
-              "MCPTool only supports header-based API key authentication."
+              "McpTool only supports header-based API key authentication."
               " Configured location:"
               f" {self._credentials_manager._auth_config.auth_scheme.in_}"
           )