_clean (and so _carries_key) now also matches each key character as %25XX,
hex in either case and for either letter case of the character, keeping
the 8+ character starts. Tests: a fully double-encoded key, recased forms,
a start of it, a mix of literal, single and double encoding, and in a
storage URL.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>