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/state/session.rs

@@ -15,6 +15,11 @@ pub(crate) struct SessionState {
     pub(crate) history: ContextManager,
     pub(crate) latest_rate_limits: Option<RateLimitSnapshot>,
     pub(crate) server_reasoning_included: bool,
+    /// Whether the session's initial context has been seeded into history.
+    ///
+    /// TODO(owen): This is a temporary solution to avoid updating a thread's updated_at
+    /// timestamp when resuming a session. Remove this once SQLite is in place.
+    pub(crate) initial_context_seeded: bool,
 }
 
 impl SessionState {
@@ -26,6 +31,7 @@ impl SessionState {
             history,
             latest_rate_limits: None,
             server_reasoning_included: false,
+            initial_context_seeded: false,
         }
     }