feat(core): add metadata field to ResponseItem (#28355)

## Description

This PR adds an optional `metadata` field to `ResponseItem` for
Responses API calls. Only mechanical plumbing, no actual values
populated and sent yet. Turns out just adding a new field to
`ResponseItem` has quite a large blast radius already.

This change is backwards compatible because `metadata` is optional and
omitted when absent, so existing response items and rollout history
without it still deserialize and requests that do not set it keep the
same wire shape. For provider compatibility, we strip out `metadata`
before non-OpenAI Responses requests so Azure and AWS Bedrock never see
this field.

My followup PR here will actually make use of it to start storing and
passing along `turn_id`: https://github.com/openai/codex/pull/28360

## What changed

- Added `ResponseItemMetadata` with optional `turn_id`, plus optional
`metadata` on Responses API item variants and inter-agent communication.
- Preserved item metadata through response-item rewrites such as
truncation, missing tool-output synthesis, compaction history
rebuilding, visible-history conversion, rollout/resume, and generated
app-server schemas/types.
- Strip item metadata from non-OpenAI Responses requests while
preserving it for OpenAI-shaped requests.
- Updated the mechanical fixture/test construction churn required by the
new optional field.

codex-rs/core/src/agent/control_tests.rs

@@ -71,6 +71,7 @@ fn assistant_message(text: &str, phase: Option<MessagePhase>) -> ResponseItem {
             text: text.to_string(),
         }],
         phase,
+        metadata: None,
     }
 }
 
@@ -90,6 +91,7 @@ fn spawn_agent_call(call_id: &str) -> ResponseItem {
         namespace: None,
         arguments: "{}".to_string(),
         call_id: call_id.to_string(),
+        metadata: None,
     }
 }
 
@@ -861,6 +863,7 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
                         text: "Parent root guidance.".to_string(),
                     }],
                     phase: None,
+                    metadata: None,
                 },
                 ResponseItem::Message {
                     id: None,
@@ -869,6 +872,7 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
                         text: "Parent subagent guidance.".to_string(),
                     }],
                     phase: None,
+                    metadata: None,
                 },
                 assistant_message("parent commentary", Some(MessagePhase::Commentary)),
                 assistant_message("parent final answer", Some(MessagePhase::FinalAnswer)),
@@ -878,6 +882,7 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
                     summary: Vec::new(),
                     content: None,
                     encrypted_content: None,
+                    metadata: None,
                 },
                 trigger_message.to_response_input_item().into(),
                 spawn_agent_call(&parent_spawn_call_id),
@@ -903,7 +908,6 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
         .flush_rollout()
         .await
         .expect("parent rollout should flush");
-
     let child_thread_id = harness
         .control
         .spawn_agent_with_metadata(
@@ -941,6 +945,7 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
                 text: "parent seed context".to_string(),
             }],
             phase: None,
+            metadata: None,
         },
         assistant_message("parent final answer", Some(MessagePhase::FinalAnswer)),
         ResponseItem::Message {
@@ -950,6 +955,7 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
                 text: "Child subagent guidance.".to_string(),
             }],
             phase: None,
+            metadata: None,
         },
     ];
     assert_eq!(
@@ -1080,6 +1086,7 @@ async fn spawn_agent_fork_strips_parent_usage_hints_from_compacted_history() {
                 text: "compacted parent summary".to_string(),
             }],
             phase: None,
+            metadata: None,
         },
         ResponseItem::Message {
             id: None,
@@ -1088,6 +1095,7 @@ async fn spawn_agent_fork_strips_parent_usage_hints_from_compacted_history() {
                 text: "Parent root guidance.".to_string(),
             }],
             phase: None,
+            metadata: None,
         },
     ];
     parent_thread
@@ -1385,6 +1393,7 @@ async fn spawn_agent_fork_last_n_turns_drops_parent_startup_prefix_when_under_li
                     text: "parent startup developer context".to_string(),
                 }],
                 phase: None,
+                metadata: None,
             }],
         )
         .await;
@@ -1506,6 +1515,7 @@ async fn spawn_agent_fork_last_n_turns_strips_parent_usage_hints() {
                         text: "Parent root guidance.".to_string(),
                     }],
                     phase: None,
+                    metadata: None,
                 },
                 spawn_agent_call(&parent_spawn_call_id),
             ],