Function qr_pairing_channel 

pub fn qr_pairing_channel(channel_type: &str) -> Option<QrPairingChannel>

Resolve a channel type key to its typed QR-pairing channel.

This is the one place a QR-login key is parsed from a string; everything downstream dispatches on QrPairingChannel. Uses the same key space as is_channel_type_compiled (kebab-case schema keys plus legacy underscore spellings, per type_keys in the compile-spec registry above — qr_pairing_keys_stay_registered_in_compile_specs pins the agreement). Returns None for channel types that have no channel-owned QR login state and for channels not compiled into this binary — the two cases callers treat identically as “unsupported”.