core: rename metadata -> internal_chat_message_metadata_passthrough (#28968)

## Description
This PR cuts Codex over from generic `ResponseItem.metadata` (introduced
here: https://github.com/openai/codex/pull/28355) to
`ResponseItem.internal_chat_message_metadata_passthrough`, which is the
blessed path and has strongly-typed keys.

For now we have to drop this MAv2 usage of `metadata`:
https://github.com/openai/codex/pull/28561 until we figure out where
that should live.

codex-rs/thread-store/src/local/mod.rs

@@ -513,7 +513,7 @@ mod tests {
                     id: None,
                     call_id: "call-1".to_string(),
                     output: FunctionCallOutputPayload::from_text("tool output".to_string()),
-                    metadata: None,
+                    internal_chat_message_metadata_passthrough: None,
                 }),
                 RolloutItem::EventMsg(EventMsg::TokenCount(
                     codex_protocol::protocol::TokenCountEvent {