Classify rollout migration failures (#39784)

## Why

Rollout migration outcomes expose free-form error messages, making failures hard to group without parsing text.

## What changed

- Add a serialized `RolloutMigrationFailureReason` to failed outcomes and classify failures across reading, conversion, SQLite materialization, publishing, and recovery.
- Add the failure reason as a low-cardinality tag on per-thread migration telemetry.

## Testing

- Cover missing SQLite metadata and invalid session metadata classifications.

GitOrigin-RevId: 2bff98483b484f4d02b72d52f39950ff99ebe429

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

@@ -25,6 +25,7 @@ pub use live_thread::LiveThread;
 pub use live_thread::LiveThreadInitGuard;
 pub use local::LocalThreadStore;
 pub use local::LocalThreadStoreConfig;
+pub use local::RolloutMigrationFailureReason;
 pub use local::RolloutMigrationMode;
 pub use local::RolloutMigrationOptions;
 pub use local::RolloutMigrationOutcome;