ci: run live suites with gpt-5.6-sol

[dimavrem22]

Executive Summary

This PR moves the real-model gateway used by live CI to gpt-5.6-sol.

• Applies the model through the shared live gateway bootstrap.
• Keeps the deterministic mock gateway unchanged.
• Explicitly passes gpt-realtime-2 to the voice gateway.

Description

The shared live bootstrap now selects openai/gpt-5.6-sol for real-model runs, covering channel, A2A, external-event, and voice workflows. The voice workflow pins gpt-realtime-2, and the bootstrap forwards that setting to the gateway process.

Reason

Scheduled full-stack coverage should exercise the requested flagship model consistently without changing deterministic mock coverage.

Decisions

• Configuration ownership: Changed the shared bootstrap so every real-model caller receives the same model without duplicating workflow configuration.

Testing

• Workflow configuration: Parsed every workflow YAML file successfully.
• Bootstrap syntax: bash -n scripts/live-aut.sh passed.
• Model configuration: Verified the real gateway selects gpt-5.6-sol and realtime voice selects gpt-realtime-2.
• Patch hygiene: git diff --check passed.