fix(mcp): Use original tool name when calling MCP server

  - Change self.name to self._mcp_tool.name in call_tool invocation
  - Ensures MCP server receives unprefixed tool name
  - Addresses review comment from alberto-mate

Author: shsha4

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

@@ -131,7 +131,7 @@ async def _run_async_impl(
     # Get the session from the session manager
     session = await self._mcp_session_manager.create_session(headers=headers)
 
-    response = await session.call_tool(self.name, arguments=args)
+    response = await session.call_tool(self._mcp_tool.name, arguments=args)
     return response
 
   async def _get_headers(