chore: save more about turn context in rollout log file (#8458)

### Motivation
- Persist richer per-turn configuration in rollouts so resumed/forked
sessions and tooling can reason about the exact instruction inputs and
output constraints used for a turn.

### Description
- Extend `TurnContextItem` to include optional `base_instructions`,
`user_instructions`, and `developer_instructions`.
- Record the optional `final_output_json_schema` associated with a turn.
- Add an optional `truncation_policy` to `TurnContextItem` and populate
it when writing turn-context rollout items.
- Introduce a protocol-level `TruncationPolicy` representation and
convert from core truncation policy when recording.

### Testing
- `cargo test -p codex-protocol` (pass)

codex-rs/protocol/src/protocol.rs

@@ -1324,6 +1324,23 @@ pub struct TurnContextItem {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub effort: Option<ReasoningEffortConfig>,
     pub summary: ReasoningSummaryConfig,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub base_instructions: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub user_instructions: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub developer_instructions: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub final_output_json_schema: Option<Value>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub truncation_policy: Option<TruncationPolicy>,
+}
+
+#[derive(Debug, Clone, Copy, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+#[serde(tag = "mode", content = "limit", rename_all = "snake_case")]
+pub enum TruncationPolicy {
+    Bytes(usize),
+    Tokens(usize),
 }
 
 #[derive(Serialize, Deserialize, Clone, JsonSchema)]