feature: Add --workers flag to nemoguardrails server for multi-worker uvicorn support

[m-misiura]

Did you check the docs?

• I have read all the NeMo-Guardrails docs

Is your feature request related to a problem? Please describe.

The server seemingly runs uvicorn with a single worker

Describe the solution you'd like

Add a --workers CLI option (defaulting to 1 for backward compatibility):

@app.command()
def server(..., workers: int = 1):
    ...
    uvicorn.run(server_app, port=port, log_level="info", host="0.0.0.0", workers=workers)

Presumably, we'd need to ensure that make api.py spawn-safe

Describe alternatives you've considered

• Gunicorn wrapper: can run gunicorn -w 4 -k uvicorn.workers.UvicornWorker nemoguardrails.server.api:app but this bypasses the CLI's config loading IIUC

Additional context

No response