refactor(auth): simplify jurisdiction mint per cleanup review

• Extract validateJurisdictionLabel, shared by targetJurisdiction and MintJurisdictionIdentityToken (was a duplicated regex + error string).
• MintJurisdictionIdentityToken returns (token, err): the audience / exchangeCore returns had no production consumer (the command discards them) and were redundant with the posted-form test assertions.
• Drop an inaccurate line from the redirectTransport test doc comment.

Behavior unchanged; fmt + lint clean, tests green.