Serve Linphone remote provisioning XML for an extension (QR code target).

Returns Linphone lpconfig provisioning XML (Content-Type: application/xml) containing the SIP account configuration for the extension bound to the given provisioning token. This endpoint is the target of the QR code generated via POST /extensions/{id}/provisioning-token; the Linphone mobile app fetches it when the QR code is scanned.

This endpoint is unauthenticated. The provisioning token acts as proof of authorization.

Note: unlike the other endpoints in this specification, this endpoint is served at /provisioning/extension WITHOUT the /v1.0 prefix.

The token query parameter must be a 64-character lowercase hex string issued by POST /extensions/{id}/provisioning-token. Tokens expire 10 minutes after issuance. Any invalid, expired, or unknown token returns HTTP 400.