On Call Transfer Destination Request

Resolve the after-hours on-call transfer (dedicated transferCall tool).

Same transfer-destination-request contract as /transfer-destination, but single-purpose: connect a new-case caller to the on-call person. The tool takes no arguments — there is exactly one destination, the customer's "On Call" TransferDestination row (for BJL a DID that rings the PBX ONCALL queue, whose membership the NetSapiens reconciler keeps equal to the scheduled on-call person).

The on-call window is re-checked here, not just at call start (the prompt's onCallActive variable is frozen when the call begins, and a call can span the window boundary). There is deliberately NO fallback chain: a refused transfer returns {"error": ...}, which plays the tool's request-failed message and lets the prompt recover into the in-house mini intake.