Align main realtime v2 flow with port branch

[aibrahim-oai]

• Make main's realtime v2 session setup, response loop, interruption flow, startup context, and playback interruption behavior match the port branch more closely while keeping the configurable model path and a single codex tool.
• Review order for the websocket split: methods_common.rs, then methods_v1.rs, then methods_v2.rs; new regression coverage stays in the e2e suites only.