Fix an issue with mounting using a FastAPI router with a root path

- Fix a bug where the mount path was not correctly appended to the root
  path of the FastAPI router
- Replace a raise statement with an assert, because the code is
  unreachable if the input attribute has correct type

Author: am1ter

fastapi_mcp/server.py

@@ -299,7 +299,9 @@ def mount(
             str,
             Doc(
                 """
-                Path where the MCP server will be mounted. Defaults to '/mcp'.
+                Path where the MCP server will be mounted.
+                Mount path is appended to the root path of FastAPI router, or to the prefix of APIRouter.
+                Defaults to '/mcp'.
                 """
             ),
         ] = "/mcp",
@@ -328,14 +330,9 @@ def mount(
             router = self.fastapi
 
         # Build the base path correctly for the SSE transport
-        if isinstance(router, FastAPI):
-            base_path = router.root_path
-        elif isinstance(router, APIRouter):
-            base_path = self.fastapi.root_path + router.prefix
-        else:
-            raise ValueError(f"Invalid router type: {type(router)}")
-
-        messages_path = f"{base_path}{mount_path}/messages/"
+        assert isinstance(router, (FastAPI, APIRouter)), f"Invalid router type: {type(router)}"
+        base_path = mount_path if isinstance(router, FastAPI) else router.prefix + mount_path
+        messages_path = f"{base_path}/messages/"
 
         sse_transport = FastApiSseTransport(messages_path)