Add collaboration_mode to TurnContextItem (#9583)

## Summary
- add optional `collaboration_mode` to `TurnContextItem` in rollouts
- persist the current collaboration mode when recording turn context
(sampling + compaction)

## Rationale
We already persist turn context data for resume logic. Capturing
collaboration mode in the rollout gives us the mode context for each
turn, enabling follow‑up work to diff mode instructions correctly on
resume.

## Changes
- protocol: add optional `collaboration_mode` field to `TurnContextItem`
- core: persist collaboration mode alongside other turn context settings
in rollouts

codex-rs/protocol/src/protocol.rs

@@ -1602,6 +1602,8 @@ pub struct TurnContextItem {
     pub approval_policy: AskForApproval,
     pub sandbox_policy: SandboxPolicy,
     pub model: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub collaboration_mode: Option<CollaborationMode>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub effort: Option<ReasoningEffortConfig>,
     pub summary: ReasoningSummaryConfig,