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/core/src/truncate.rs

@@ -6,6 +6,7 @@ use crate::config::Config;
 use codex_protocol::models::FunctionCallOutputContentItem;
 use codex_protocol::openai_models::TruncationMode;
 use codex_protocol::openai_models::TruncationPolicyConfig;
+use codex_protocol::protocol::TruncationPolicy as ProtocolTruncationPolicy;
 
 const APPROX_BYTES_PER_TOKEN: usize = 4;
 
@@ -15,6 +16,15 @@ pub enum TruncationPolicy {
     Tokens(usize),
 }
 
+impl From<TruncationPolicy> for ProtocolTruncationPolicy {
+    fn from(value: TruncationPolicy) -> Self {
+        match value {
+            TruncationPolicy::Bytes(bytes) => Self::Bytes(bytes),
+            TruncationPolicy::Tokens(tokens) => Self::Tokens(tokens),
+        }
+    }
+}
+
 impl From<TruncationPolicyConfig> for TruncationPolicy {
     fn from(config: TruncationPolicyConfig) -> Self {
         match config.mode {