feat(otel, core): record turn TTFT and TTFM metrics in codex-core (#13630)

### Summary
This adds turn-level latency metrics for the first model output and the
first completed agent message.
- `codex.turn.ttft.duration_ms` starts at turn start and records on the
first output signal we see from the model. That includes normal
assistant text, reasoning deltas, and non-text outputs like tool-call
items.
- `codex.turn.ttfm.duration_ms` also starts at turn start, but it
records when the first agent message finishes streaming rather than when
its first delta arrives.

### Implementation notes
The timing is tracked in codex-core, not app-server, so the definition
stays consistent across CLI, TUI, and app-server clients.

I reused the existing turn lifecycle boundary that already drives
`codex.turn.e2e_duration_ms`, stored the turn start timestamp in turn
state, and record each metric once per turn.

I also wired the new metric names into the OTEL runtime metrics summary
so they show up in the same in-memory/debug snapshot path as the
existing timing metrics.

codex-rs/tui/src/history_cell.rs

@@ -2580,6 +2580,8 @@ mod tests {
             responses_api_engine_service_ttft_ms: 460,
             responses_api_engine_iapi_tbt_ms: 1_180,
             responses_api_engine_service_tbt_ms: 1_240,
+            turn_ttft_ms: 0,
+            turn_ttfm_ms: 0,
         };
         let cell = FinalMessageSeparator::new(Some(12), Some(summary));
         let rendered = render_lines(&cell.display_lines(600));