Reuse current model context across paginated resume config loading (#56224)

## Why

Resuming a paginated thread rechecks stored history after loading configuration. Unchanged model context can be reused instead of loading it again.

## What changed

Retain the prepared model context across configuration loading and reuse it when the thread ID, rollout path, and history revision still match. Add `ThreadStore::is_model_context_current` to validate revisions, with a fresh read when validation fails or is unavailable.

## Testing

Add coverage for revision invalidation after history is appended or removed. Extend resume tests to cover paginated history, concurrent metadata and lifecycle changes, workspace-root changes, and preservation of saved user messages in model requests.

GitOrigin-RevId: bad2ca0caddb63523264ac530ffb801db3961f2b

codex-rs/thread-store/src/store.rs

@@ -268,6 +268,19 @@ pub trait ThreadStore: Any + Send + Sync {
         })
     }
 
+    /// Checks whether a previously loaded model context still represents the stored history.
+    ///
+    /// Stores that cannot validate the opaque revision must return false. A false result or an
+    /// error requires a fresh read. This does not acquire writer ownership;
+    /// [`Self::resume_thread`] must still validate the snapshot after acquisition.
+    fn is_model_context_current(
+        &self,
+        _params: LoadThreadHistoryParams,
+        _revision: String,
+    ) -> ThreadStoreFuture<'_, bool> {
+        Box::pin(async { Ok(false) })
+    }
+
     /// Freezes source history and model context used to initialize a referenced fork.
     ///
     /// Stores without reference-backed fork support can retain this default implementation.