env_prefix falls back to env vars without prefix

[hofrob]

Using my example from #437: https://github.com/hofrob/pydantic-settings-env-prefix

If you uv run hello the result is:

hello='ABC'

This value is coming from the first line in .env: HELLO=ABC

I don't think the intention of env_prefix is to just fall back to whatever env var it can find that matches the variable without the prefix. If you have nested or complex settings with multiple API keys managed via different env_prefix values, you'll not want to use an API key with the wrong service. I'd consider this API key compromised.

See also this comment: #437 (comment)