test: preserve MCPServer branch coverage

Author: he-yufeng

src/mcp/server/mcpserver/server.py

@@ -297,7 +297,7 @@ def run(
                     anyio.run(self.run_stdio_async)
                 case "sse":
                     anyio.run(lambda: self.run_sse_async(**kwargs))
-                case "streamable-http":
+                case "streamable-http":  # pragma: no branch
                     anyio.run(lambda: self.run_streamable_http_async(**kwargs))
         except KeyboardInterrupt:
             return