chore: add phase to message responseitem (#10455)

### What

add wiring for `phase` field on `ResponseItem::Message` to lay
groundwork for differentiating model preambles and final messages.
currently optional.

follows pattern in #9698.

updated schemas with `just write-app-server-schema` so we can see type
changes.

### Tests
Updated existing tests for SSE parsing and hydrating from history

codex-rs/app-server/tests/suite/v2/compaction.rs

@@ -126,6 +126,7 @@ async fn auto_compaction_remote_emits_started_and_completed_items() -> Result<()
                 text: "REMOTE_COMPACT_SUMMARY".to_string(),
             }],
             end_turn: None,
+            phase: None,
         },
         ResponseItem::Compaction {
             encrypted_content: "ENCRYPTED_COMPACTION_SUMMARY".to_string(),