📝 Add clear error message for missing route tags in app/main.py.

[rxxbyy]

Added a guard statement with a clear error message when a route is missing tags.

Currently, when you miss setting up a tag you get an error like:

return f"{route.tags[0]}-{route.name}"
          ~~~~~~~~~~^^^
IndexError: list index out of range

With the proposed changes, the developer sees:

ValueError: Route 'my_new_route' is missing a tag. Each route must have at least one tag for client generation.

I believe this improves developer experience and saves debugging time for anyone using the template.
Thank you!