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/core/tests/suite/compact.rs

@@ -1452,6 +1452,7 @@ async fn auto_compact_runs_after_resume_when_token_usage_is_over_limit() {
                 text: remote_summary.to_string(),
             }],
             end_turn: None,
+            phase: None,
         },
         codex_protocol::models::ResponseItem::Compaction {
             encrypted_content: "ENCRYPTED_COMPACTION_SUMMARY".to_string(),
@@ -2275,6 +2276,7 @@ async fn auto_compact_counts_encrypted_reasoning_before_last_user() {
                 text: "REMOTE_COMPACT_SUMMARY".to_string(),
             }],
             end_turn: None,
+            phase: None,
         },
         codex_protocol::models::ResponseItem::Compaction {
             encrypted_content: "ENCRYPTED_COMPACTION_SUMMARY".to_string(),
@@ -2395,6 +2397,7 @@ async fn auto_compact_runs_when_reasoning_header_clears_between_turns() {
                 text: "REMOTE_COMPACT_SUMMARY".to_string(),
             }],
             end_turn: None,
+            phase: None,
         },
         codex_protocol::models::ResponseItem::Compaction {
             encrypted_content: "ENCRYPTED_COMPACTION_SUMMARY".to_string(),