fix(app-server, core): defer initial context write to rollout file until first turn (#9950)

### Overview
Currently calling `thread/resume` will always bump the thread's
`updated_at` timestamp. This PR makes it the `updated_at` timestamp
changes only if a turn is triggered.

### Additonal context
What we typically do on resuming a thread is **always** writing “initial
context” to the rollout file immediately. This initial context includes:
- Developer instructions derived from sandbox/approval policy + cwd
- Optional developer instructions (if provided)
- Optional collaboration-mode instructions
- Optional user instructions (if provided)
- Environment context (cwd, shell, etc.)

This PR defers writing the “initial context” to the rollout file until
the first `turn/start`, so we don't inadvertently bump the thread's
`updated_at` timestamp until a turn is actually triggered.

This works even though both `thread/resume` and `turn/start` accept
overrides (such as `model`, `cwd`, etc.) because the initial context is
seeded from the effective `TurnContext` in memory, computed at
`turn/start` time, after both sets of overrides have been applied.

**NOTE**: This is a very short-lived solution until we introduce sqlite.
Then we can remove this.

codex-rs/core/src/tasks/mod.rs

@@ -115,6 +115,8 @@ impl Session {
         task: T,
     ) {
         self.abort_all_tasks(TurnAbortReason::Replaced).await;
+        self.seed_initial_context_if_needed(turn_context.as_ref())
+            .await;
 
         let task: Arc<dyn SessionTask> = Arc::new(task);
         let task_kind = task.kind();