[codex] Start the guardian child session when parent session is started (#27982)

## Why

The first auto-review currently creates its Guardian child session on
demand, adding avoidable latency before the review can begin. Creating
the ordinary Guardian child during parent-session initialization lets
that child use the existing session startup WebSocket prewarm before the
first escalation. This does not introduce a Guardian-specific prewarm
mechanism.

## What changed

- initialize the existing Guardian review-session manager owned by
`Session` when a thread starts with auto-review enabled and an approval
policy that routes to Guardian
- use the standard Guardian child-session construction and the existing
session startup WebSocket prewarm
- preserve the existing reuse-key invalidation and lazy creation
fallback when startup initialization fails or the effective review
configuration changes
- add an integration test that verifies normal root-session startup
emits a Guardian `generate=false` prewarm request

## Benchmark

I compared release builds against main. Each prompt first ran a
non-escalated `sleep 3`, then requested an escalated marker command.

| binary | count | avg Guardian duration | median Guardian duration |
avg Guardian TTFT |
|---|---:|---:|---:|---:|
| origin-main | 10 | 4008.7 ms | 3949.5 ms | 3746.5 ms |
| session-fix | 10 | 2865.0 ms | 2594.0 ms | 2492.7 ms |

Guardian duration fell by 28.5% and Guardian TTFT fell by 33.5%. These
measurements cover Guardian review latency; they do not measure parent
thread-start latency.

codex-rs/core/src/session/session.rs

@@ -1217,7 +1217,6 @@ impl Session {
                 let mut state = sess.state.lock().await;
                 state.queue_pending_session_start_source(session_start_source);
             }
-
             Ok(sess)
         }
         .await;