Track multi-agent mode in world state (#34845)

## Why

Multi-agent mode instructions are durable model context and need to survive
history changes without re-emitting unrelated setup hints.

## What changed

- Add a `multi_agent_mode` world-state section that diffs, retains, and restores
  the effective mode instructions.
- Persist mode changes in world-state snapshots while keeping the legacy
  `TurnContextItem.multi_agent_mode` field for reading older rollouts.
- Keep initial mode instructions after the root-agent usage hint so the active
  mode takes precedence, and bound custom mode text before snapshotting it.

## Testing

- Add snapshot coverage for mode transitions, retained history, and custom-text
  truncation.
- Verify live mode changes preserve instruction ordering, avoid duplicating the
  usage hint, and record both modes in world state.

GitOrigin-RevId: 96b9964d6e5299b07a011c559b68e3d948a49226

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

@@ -389,7 +389,7 @@ impl TurnContext {
             personality: self.personality,
             collaboration_mode: Some(self.collaboration_mode()),
             multi_agent_version: Some(self.multi_agent_version),
-            multi_agent_mode: super::multi_agents::effective_multi_agent_mode(self),
+            multi_agent_mode: None,
             realtime_active: Some(self.realtime_active),
             effort: self.reasoning_effort.clone(),
             summary: ReasoningSummaryConfig::Auto,