Run compact session-start hooks before turn continuation (#34396)

## Why

Mid-turn auto-compaction queued `SessionStart` hooks but continued sampling
before running them. This delayed hook-provided context and ignored requests to
stop the continuation.

## What changed

Drain pending session-start hooks immediately after successful mid-turn
auto-compaction. End the turn when a hook requests a stop; otherwise include
its additional context in the next sampling request.

## Testing

Added coverage for repeated compactions in one turn, context delivery without
leaking hooks into the next user turn, and stop requests that block sampling.

GitOrigin-RevId: c57708a792fb47d98d95c38d7d91bcd9f235be84

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

@@ -371,6 +371,9 @@ pub(crate) async fn run_turn(
                             .await;
                         return Ok(None);
                     }
+                    if run_pending_session_start_hooks(&sess, &turn_context).await {
+                        return Ok(None);
+                    }
                     can_drain_pending_input = !model_needs_follow_up;
                     continue;
                 }