Defer persistence of rollout file (#11028)

- Defer rollout persistence for fresh threads (`InitialHistory::New`):
keep rollout events in memory and only materialize rollout file + state
DB row on first `EventMsg::UserMessage`.
- Keep precomputed rollout path available before materialization.
- Change `thread/start` to build thread response from live config
snapshot and optional precomputed path.
- Improve pre-materialization behavior in app-server/TUI: clearer
invalid-request errors for file-backed ops and a friendlier `/fork` “not
ready yet” UX.
- Update tests to match deferred semantics across
start/read/archive/unarchive/fork/resume/review flows.
- Improved resilience of user_shell test, which should be unrelated to
this change but must be affected by timing changes

For Reviewers:
* The primary change is in recorder.rs
* Most of the other changes were to fix up broken assumptions in
existing tests

Testing:
* Manually tested CLI
* Exercised app server paths by manually running IDE Extension with
rebuilt CLI binary
* Only user-visible change is that `/fork` in TUI generates visible
error if used prior to first turn

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

@@ -43,6 +43,18 @@ async fn new_thread_is_recorded_in_state_db() -> Result<()> {
     }
 
     let db = test.codex.state_db().expect("state db enabled");
+    assert!(
+        !rollout_path.exists(),
+        "fresh thread rollout should not be materialized before first user message"
+    );
+
+    let initial_metadata = db.get_thread(thread_id).await?;
+    assert!(
+        initial_metadata.is_none(),
+        "fresh thread should not be recorded in state db before first user message"
+    );
+
+    test.submit_turn("materialize rollout").await?;
 
     let mut metadata = None;
     for _ in 0..100 {
@@ -56,6 +68,10 @@ async fn new_thread_is_recorded_in_state_db() -> Result<()> {
     let metadata = metadata.expect("thread should exist in state db");
     assert_eq!(metadata.id, thread_id);
     assert_eq!(metadata.rollout_path, rollout_path);
+    assert!(
+        rollout_path.exists(),
+        "rollout should be materialized after first user message"
+    );
 
     Ok(())
 }