Validate an authorized B2 native API endpoint URL.
Parameters
raw: string
URL string from B2 authorization.
Returns string|null
null when the URL is trusted, otherwise a human-readable reason.
Remarks
B2 authorize responses provide the native API origin that subsequent raw SDK
calls use. The server accepts only HTTPS Backblaze-owned origins without
credentials, custom ports, paths, queries, or fragments.
Validate an authorized B2 native API endpoint URL.