[codex-analytics] guardian review TTFT plumbing and emission (#17696)

## Why

Guardian analytics includes time-to-first-token, but the Guardian
reviewer runs as a normal Codex session and `TurnCompleteEvent` did not
expose TTFT. The timing needs to flow through the standard
turn-completion protocol so Guardian review analytics can consume the
same value as the rest of the session machinery.

## What changed

Adds optional `time_to_first_token_ms` to `TurnCompleteEvent` and
populates it from `TurnTiming`. The value is carried through app-server
thread history, rollout reconstruction, TUI/app-server adapters, and
Guardian review session handling.

Guardian review analytics now captures TTFT from the reviewer
turn-complete event when available. Existing tests and fixtures are
updated to set the new optional field to `None` where TTFT is not
relevant.

## Verification

- `cargo clippy -p codex-tui --tests -- -D warnings`
- `cargo clippy -p codex-core --lib --tests -- -D warnings`

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/17696).
* __->__ #17696
* #17695
* #17693
* #18278
* #18953

codex-rs/core/src/tools/handlers/multi_agents_tests.rs

@@ -1155,6 +1155,7 @@ async fn multi_agent_v2_list_agents_returns_completed_status_and_last_task_messa
                 last_agent_message: Some("done".to_string()),
                 completed_at: None,
                 duration_ms: None,
+                time_to_first_token_ms: None,
             }),
         )
         .await;
@@ -1633,6 +1634,7 @@ async fn multi_agent_v2_followup_task_completion_notifies_parent_on_every_turn()
                 last_agent_message: Some("first done".to_string()),
                 completed_at: None,
                 duration_ms: None,
+                time_to_first_token_ms: None,
             }),
         )
         .await;
@@ -1661,6 +1663,7 @@ async fn multi_agent_v2_followup_task_completion_notifies_parent_on_every_turn()
                 last_agent_message: Some("second done".to_string()),
                 completed_at: None,
                 duration_ms: None,
+                time_to_first_token_ms: None,
             }),
         )
         .await;