Root cause of the 401 was a corrupted API key: the WordPress auth
salt changed after the key was encrypted, so decryption produced
garbage instead of sk-or-... OpenRouter interprets the garbled
Bearer token as "Missing Authentication header."
Adds the same sk-or- prefix check to the credential validation
method so users see a clear "re-enter your key" message instead
of a cryptic 401 error.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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