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/core/src/client_tests.rs

@@ -226,7 +226,7 @@ fn output_message(id: &str, text: &str) -> ResponseItem {
             text: text.to_string(),
         }],
         phase: None,
-        metadata: None,
+        internal_chat_message_metadata_passthrough: None,
     }
 }