#11103: voice-call: improve Telnyx transcription + auto-responses

Summary - Telnyx: normalize call.transcription more defensively and avoid losing call association when client_state is missing. - Telnyx: auto-start transcription on call.answered in conversation mode. - Webhook transcripts: debounce and suppress duplicates/partials; handle barge-in and superseding in-flight generations. - Response generator: allow selecting response agent via responseAgentId and response session key prefix. Follow-ups - Webhook ctx.url no longer trusts Host header (uses config.publicUrl when available). - Telnyx is_final defaults to true when missing. Test plan - Place a Telnyx call (conversation mode) and speak a short question. - Confirm transcription events map to the active call and you get a single spoken response (no duplicates).