Preserve the resolved multi-agent version when reverting threads (#43000)

## Why

Reverting a paginated thread can remove the turn context that records its resolved multi-agent version. Preserve that version so a model-selected V2 thread keeps its collaboration tools after reload or a server restart.

## What changed

Pass the live thread's `multi_agent_version` into the thread store and persist it in the replacement rollout's session metadata. When no live version is supplied, retain the existing metadata value. Document this guarantee for `thread/revert`.

## Testing

Add regression tests for model-selected V2 across live reload and cold resume, plus metadata preservation for `V1`, `V2`, and `Disabled` across repeated reverts.

GitOrigin-RevId: bd7d4d76a50d23da16caa2a0631acf10bfa49629

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -2189,6 +2189,7 @@ impl ThreadRequestProcessor {
             .revert_thread(codex_thread_store::RevertThreadParams {
                 thread_id,
                 before_turn_id,
+                multi_agent_version: thread.multi_agent_version(),
             })
             .await
             .map_err(|err| thread_store_mutation_error("revert", err));