feat(toolmgr): add get_tool_by_name for unified tool lookup

Add unified tool lookup method that searches both plugin and MCP loaders.
Also add _get_tool method to MCPLoader for consistency with PluginToolLoader.

src/langbot/pkg/provider/tools/loaders/mcp.py

@@ -384,6 +384,21 @@ class MCPLoader(loader.ToolLoader):
                     return True
         return False
 
+    async def _get_tool(self, name: str) -> resource_tool.LLMTool | None:
+        """Get tool by name.
+
+        Args:
+            name: Tool name to find
+
+        Returns:
+            LLMTool if found, None otherwise
+        """
+        for session in self.sessions.values():
+            for function in session.get_tools():
+                if function.name == name:
+                    return function
+        return None
+
     async def invoke_tool(self, name: str, parameters: dict, query: pipeline_query.Query) -> typing.Any:
         """执行工具调用"""
         for session in self.sessions.values():