Use testing.Testing() inside createTransport to automatically
clone http.DefaultTransport when running in tests. In production,
DefaultTransport is used as-is (efficient connection pooling).
This fixes the CloseIdleConnections flake class: httptest.Server.Close()
calls http.DefaultTransport.CloseIdleConnections(), which disrupts
any MCP client sharing that transport. The testing.Testing() check
means every MCP transport created during tests gets isolation
automatically, with no caller changes needed.
Closescoder/internal#1016
Closes PLAT-291
Mathias Fredriksson