## Why
Voice failure metrics lacked cause and lifecycle phase, and session durations were recorded before the final outcome was known. A failed control send could also overwrite a more specific WebRTC error.
## What changed
- Tag `codex.voice.session.failure` with bounded `cause` and `phase` values, recording at most one failure per session.
- Capture duration when stopping and emit `codex.voice.session.duration` at cleanup with a `success` or `failure` outcome.
- Carry typed WebRTC errors into the TUI, preserve existing errors when control sends fail, and avoid duplicate errors after startup failure.
- Record app-server disconnect failures for background voice sessions as well as the visible session.
## Testing
Add coverage for terminal outcome and failure metric attributes, preservation of specific startup and audio-session errors, and startup failures without duplicate control-channel errors.
GitOrigin-RevId: 5aeda6e766093e3fce7206560aa83b6f7770dddb
Benjamin Carlsson
322bbf4d84