[codex] Trim unused TurnContextItem fields (#22709)

## Why

`TurnContextItem` is the durable baseline used to reconstruct context
diffs across resume/fork. Most of the old persisted-only fields on it
are no longer read, so keeping them in rollout snapshots adds schema
surface and state that can drift without affecting reconstruction.

`summary` is the exception: older Codex versions require it to
deserialize `turn_context` records, so keep writing a default
compatibility value until that schema surface can be removed safely.

## What changed

- Removed the unused persisted fields from `TurnContextItem`: trace ids,
user/developer instructions, output schema, and truncation policy.
- Kept `summary` with a compatibility comment and made
`TurnContext::to_turn_context_item` write `ReasoningSummary::Auto`
instead of live turn state.
- Updated rollout/context reconstruction fixtures for the retained
summary field.

## Verification

- `cargo test -p codex-protocol --lib turn_context_item`
- `cargo test -p codex-rollout
resume_candidate_matches_cwd_reads_latest_turn_context`
- `cargo test -p codex-state turn_context`
- `cargo test -p codex-core --lib
new_default_turn_captures_current_span_trace_id`
- `cargo test -p codex-core --lib
record_initial_history_resumed_turn_context_after_compaction_reestablishes_reference_context_item`
- `cargo test -p codex-core --test all
emits_warning_when_resumed_model_differs`
- `git diff --check`

codex-rs/protocol/src/protocol.rs

@@ -2824,8 +2824,6 @@ pub struct TurnContextNetworkItem {
 pub struct TurnContextItem {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub turn_id: Option<String>,
-    #[serde(default, skip_serializing_if = "Option::is_none")]
-    pub trace_id: Option<String>,
     pub cwd: PathBuf,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub current_date: Option<String>,
@@ -2848,15 +2846,11 @@ pub struct TurnContextItem {
     pub realtime_active: Option<bool>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub effort: Option<ReasoningEffortConfig>,
+    // Compatibility-only field written with a default value so older Codex
+    // versions can deserialize turn-context rollout items. It is no longer
+    // read by context reconstruction and should be removed in a future schema
+    // cleanup.
     pub summary: ReasoningSummaryConfig,
-    #[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>,
 }
 
 impl TurnContextItem {
@@ -5246,7 +5240,6 @@ mod tests {
             "summary": "auto",
         }))?;
 
-        assert_eq!(item.trace_id, None);
         assert_eq!(item.network, None);
         assert_eq!(item.file_system_sandbox_policy, None);
         Ok(())
@@ -5256,7 +5249,6 @@ mod tests {
     fn turn_context_item_serializes_network_when_present() -> Result<()> {
         let item = TurnContextItem {
             turn_id: None,
-            trace_id: None,
             cwd: test_path_buf("/tmp"),
             current_date: None,
             timezone: None,
@@ -5281,10 +5273,6 @@ mod tests {
             realtime_active: None,
             effort: None,
             summary: ReasoningSummaryConfig::Auto,
-            user_instructions: None,
-            developer_instructions: None,
-            final_output_json_schema: None,
-            truncation_policy: None,
         };
 
         let value = serde_json::to_value(item)?;
@@ -5308,6 +5296,7 @@ mod tests {
                 }]
             })
         );
+        assert_eq!(value["summary"], json!("auto"));
         Ok(())
     }