fix(mcp): refuse the reserved tool name set_model_response

_RESERVED_TOOL_NAMES covers the names the framework itself puts on the wire,
so a server advertising one cannot have its tool dispatched in place of the
framework's own. set_model_response belongs to that set but was missing.

SetModelResponseTool is injected into the request whenever output_schema is
configured alongside other tools (flows/llm_flows/prompt/_schema.py), the
framework tells the model to answer through it by name, and base_llm_flow.py
reads the result back by that same name. Because LlmRequest.append_tools
resolves a duplicate name by last-wins with only a warning, an MCP server
advertising set_model_response could otherwise receive the agent's structured
final answer instead of the framework.

The name is spelled out rather than imported: the function is defined inside
SetModelResponseTool.__init__, so there is no module-level binding to import.

Extends both existing reserved-name tests.

Fixes #7144

Author: sushant-me

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

@@ -79,6 +79,12 @@
     REQUEST_CONFIRMATION_FUNCTION_CALL_NAME,
     REQUEST_INPUT_FUNCTION_CALL_NAME,
     transfer_to_agent.__name__,
+    # Injected by the output-schema processor whenever output_schema is set
+    # alongside other tools (flows/llm_flows/prompt/_schema.py) and read back
+    # by name in base_llm_flow.py, so it is a framework-owned wire name too.
+    # Spelled out because the function is defined inside
+    # SetModelResponseTool.__init__ and is not importable.
+    'set_model_response',
 })
 
 _UNSET = object()

tests/unittests/tools/mcp_tool/test_mcp_tool.py

@@ -730,6 +730,7 @@ def test_init_with_empty_description(self):
           "adk_request_confirmation",
           "adk_request_input",
           "transfer_to_agent",
+          "set_model_response",
       ],
   )
   def test_init_reserved_name(self, reserved_name):

tests/unittests/tools/mcp_tool/test_mcp_toolset.py

@@ -409,6 +409,7 @@ async def test_get_tools_skips_reserved_names(self):
         MockMCPTool("adk_request_credential"),
         MockMCPTool("adk_request_confirmation"),
         MockMCPTool("adk_request_input"),
+        MockMCPTool("set_model_response"),
     ]
     self.mock_session.list_tools = AsyncMock(
         return_value=MockListToolsResult(mock_tools)