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/tests/suite/resume_warning.rs

@@ -74,7 +74,7 @@ fn resume_history(
                 time_to_first_token_ms: None,
             })),
         ],
-        rollout_path: rollout_path.to_path_buf(),
+        rollout_path: Some(rollout_path.to_path_buf()),
     })
 }