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/codex-api/src/endpoint/responses_websocket.rs

@@ -825,7 +825,7 @@ mod tests {
                     text: "hello".to_string(),
                 }],
                 phase: None,
-                metadata: None,
+                internal_chat_message_metadata_passthrough: None,
             }],
             tools: vec![json!({
                 "type": "function",