Enforce single-line secrets

[nml0]

Problem Description

Ensure authentication secrets (username, api_token, pat) are strictly single-line values. This prevents accidental multiline pastes (common with tokens copied from browsers or password managers)

Proposed Solution

I propos to add a reusable SingleLineSecret type using Pydantic v2 Annotated + AfterValidator.
If desirable, I'd like to create a PR for this.