Function validate_identity_component 

pub fn validate_identity_component(field: &str, value: &str) -> ValidationResult

Validate a resolved kopia identity component (username/hostname). Shape-only (see [identity_char_problem]); field names the surface for the message. Called both from the static admission validator (on explicit overrides) and from crate::resolve_identity (on the fully-resolved value, covering CEL results and defaults), so a bad identity can never be pinned.