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/codex_delegate_tests.rs

@@ -81,7 +81,7 @@ async fn forward_events_cancelled_while_send_blocked_shuts_down_delegate() {
                     call_id: "call-1".to_string(),
                     name: "tool".to_string(),
                     input: "{}".to_string(),
-                    metadata: None,
+                    internal_chat_message_metadata_passthrough: None,
                 },
             }),
         })