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_thread.rs

@@ -452,7 +452,7 @@ impl CodexThread {
             role: "user".to_string(),
             content: vec![ContentItem::InputText { text: message }],
             phase: None,
-            metadata: None,
+            internal_chat_message_metadata_passthrough: None,
         };
         self.codex
             .session