[codex] Stabilize subagent start hook test (#23882)

## What

Remove the exact captured request-count assertion from the
`SubagentStart` hook integration test while still waiting for the child
request that matches the injected hook context.

## Why

The test owns the start-hook behavior and already verifies that the
child request reaches the context matcher plus that the start/session
hook logs have the expected invocations. Counting every request captured
by the response mock makes the test sensitive to lifecycle timing
outside that contract and has been flaky in CI.

## Testing

- `cargo test -p codex-core --test all
suite::subagent_notifications::subagent_start_replaces_session_start_and_injects_context
-- --exact`

codex-rs/core/tests/suite/subagent_notifications.rs

@@ -519,8 +519,7 @@ async fn subagent_start_replaces_session_start_and_injects_context() -> Result<(
         .await?;
 
     test.submit_turn(TURN_1_PROMPT).await?;
-    let child_requests = wait_for_requests(&child_request_log).await?;
-    assert_eq!(child_requests.len(), 1);
+    let _ = wait_for_requests(&child_request_log).await?;
 
     let start_inputs = wait_for_hook_log(
         test.codex_home_path(),