Hi @Swapnil_Marathe, tbh I am not quite sure what the expected behaviour for the Swagger UI would be if running n8n via npm run dev.
Is the actual API working for you? If so, you could simply check the reference at API reference - n8n Documentation to get an idea of the available endpoints.
Or alternatively fire up a temporary n8n instance via docker run --rm -p 5678:5678 n8nio/n8n:0.186.0 to quickly take a look at Swagger locally under http://localhost:5678/api/v1/docs/.
I have just tested this and it appears to be working correctly, The one thing I have noticed is you are using a differnet port, If you change that back to 5678 does it change anything?
Do you have anything else running like some kind of local proxy for testing?