Migrate fork and resume reads to thread store (#18900)

- Route cold thread/resume and thread/fork source loading through
ThreadStore reads instead of direct rollout path operations
- Keep lookups that explicitly specify a rollout-path using the local
thread store methods but return an invalid-request error for remote
ThreadStore configurations
- Add some additional unit tests for code path coverage

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

@@ -1268,7 +1268,7 @@ async fn record_initial_history_reconstructs_resumed_transcript() {
         .record_initial_history(InitialHistory::Resumed(ResumedHistory {
             conversation_id: ThreadId::default(),
             history: rollout_items,
-            rollout_path: PathBuf::from("/tmp/resume.jsonl"),
+            rollout_path: Some(PathBuf::from("/tmp/resume.jsonl")),
         }))
         .await;
 
@@ -1297,7 +1297,7 @@ async fn resumed_history_injects_initial_context_on_first_context_update_only()
         .record_initial_history(InitialHistory::Resumed(ResumedHistory {
             conversation_id: ThreadId::default(),
             history: rollout_items,
-            rollout_path: PathBuf::from("/tmp/resume.jsonl"),
+            rollout_path: Some(PathBuf::from("/tmp/resume.jsonl")),
         }))
         .await;
 
@@ -1390,7 +1390,7 @@ async fn record_initial_history_seeds_token_info_from_rollout() {
         .record_initial_history(InitialHistory::Resumed(ResumedHistory {
             conversation_id: ThreadId::default(),
             history: rollout_items,
-            rollout_path: PathBuf::from("/tmp/resume.jsonl"),
+            rollout_path: Some(PathBuf::from("/tmp/resume.jsonl")),
         }))
         .await;