Follow-up to #12704. The SignalAdapter can resolve +E164 numbers to
UUIDs via listContacts, but _parse_target_ref() in the send_message
tool rejected '+' as non-digit and fell through to channel-name
resolution — which fails for contacts without a prior session entry.
Adds an E.164 branch in _parse_target_ref for phone-based platforms
(signal, sms, whatsapp) that preserves the leading '+' so downstream
adapters keep the format they expect. Non-phone platforms are
unaffected.
Reported by @qdrop17 on Discord after pulling #12704.
be472138f3