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/app-server/tests/suite/v2/compaction.rs

@@ -136,9 +136,11 @@ async fn auto_compaction_remote_emits_started_and_completed_items() -> Result<()
                 text: "REMOTE_COMPACT_SUMMARY".to_string(),
             }],
             phase: None,
+            metadata: None,
         },
         ResponseItem::Compaction {
             encrypted_content: "ENCRYPTED_COMPACTION_SUMMARY".to_string(),
+            metadata: None,
         },
     ];
     let compact_mock = responses::mount_compact_json_once(

codex-rs/app-server/tests/suite/v2/thread_inject_items.rs

@@ -60,6 +60,7 @@ async fn thread_inject_items_adds_raw_response_items_to_thread_history() -> Resu
             text: injected_text.to_string(),
         }],
         phase: None,
+        metadata: None,
     };
 
     let inject_req = mcp
@@ -197,6 +198,7 @@ async fn thread_inject_items_adds_raw_response_items_after_a_turn() -> Result<()
             text: "Injected after first turn".to_string(),
         }],
         phase: None,
+        metadata: None,
     };
     let injected_value = serde_json::to_value(&injected_item)?;
 

codex-rs/app-server/tests/suite/v2/thread_resume.rs

@@ -2449,6 +2449,7 @@ async fn thread_resume_rejects_history_when_thread_is_running() -> Result<()> {
                     text: "history override".to_string(),
                 }],
                 phase: None,
+                metadata: None,
             }]),
             ..Default::default()
         })
@@ -3456,6 +3457,7 @@ async fn thread_resume_supports_history_and_overrides() -> Result<()> {
             text: history_text.to_string(),
         }],
         phase: None,
+        metadata: None,
     }];
 
     // Resume with explicit history and override the model.