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/client.rs

@@ -29,6 +29,7 @@ use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::Settings;
 use codex_protocol::config_types::Verbosity;
 use codex_protocol::models::FunctionCallOutputPayload;
+use codex_protocol::models::MessagePhase;
 use codex_protocol::models::ReasoningItemContent;
 use codex_protocol::models::ReasoningItemReasoningSummary;
 use codex_protocol::models::WebSearchAction;
@@ -197,6 +198,7 @@ async fn resume_includes_initial_messages_and_sends_prior_items() {
             text: "resumed user message".to_string(),
         }],
         end_turn: None,
+        phase: None,
     };
     let prior_user_json = serde_json::to_value(&prior_user).unwrap();
     writeln!(
@@ -218,6 +220,7 @@ async fn resume_includes_initial_messages_and_sends_prior_items() {
             text: "resumed system instruction".to_string(),
         }],
         end_turn: None,
+        phase: None,
     };
     let prior_system_json = serde_json::to_value(&prior_system).unwrap();
     writeln!(
@@ -239,6 +242,7 @@ async fn resume_includes_initial_messages_and_sends_prior_items() {
             text: "resumed assistant message".to_string(),
         }],
         end_turn: None,
+        phase: Some(MessagePhase::Commentary),
     };
     let prior_item_json = serde_json::to_value(&prior_item).unwrap();
     writeln!(
@@ -318,6 +322,25 @@ async fn resume_includes_initial_messages_and_sends_prior_items() {
         .iter()
         .position(|(role, text)| role == "assistant" && text == "resumed assistant message")
         .expect("prior assistant message");
+    let prior_assistant = input
+        .iter()
+        .find(|item| {
+            item.get("role").and_then(|role| role.as_str()) == Some("assistant")
+                && item
+                    .get("content")
+                    .and_then(|content| content.as_array())
+                    .and_then(|content| content.first())
+                    .and_then(|entry| entry.get("text"))
+                    .and_then(|text| text.as_str())
+                    == Some("resumed assistant message")
+        })
+        .expect("resumed assistant message request item");
+    assert_eq!(
+        prior_assistant
+            .get("phase")
+            .and_then(|phase| phase.as_str()),
+        Some("commentary")
+    );
     let pos_permissions = messages
         .iter()
         .position(|(role, text)| role == "developer" && text.contains("<permissions instructions>"))
@@ -1210,6 +1233,7 @@ async fn azure_responses_request_includes_store_and_reasoning_ids() {
             text: "message".into(),
         }],
         end_turn: None,
+        phase: None,
     });
     prompt.input.push(ResponseItem::WebSearchCall {
         id: Some("web-search-id".into()),