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/codex-api/src/requests/chat.rs

@@ -387,6 +387,7 @@ mod tests {
                 text: "hi".to_string(),
             }],
             end_turn: None,
+            phase: None,
         }];
         let req = ChatRequestBuilder::new("gpt-test", "inst", &prompt_input, &[])
             .conversation_id(Some("conv-1".into()))
@@ -414,6 +415,7 @@ mod tests {
                     text: "read these".to_string(),
                 }],
                 end_turn: None,
+                phase: None,
             },
             ResponseItem::FunctionCall {
                 id: None,

codex-rs/codex-api/src/requests/responses.rs

@@ -224,12 +224,14 @@ mod tests {
                 role: "assistant".into(),
                 content: Vec::new(),
                 end_turn: None,
+                phase: None,
             },
             ResponseItem::Message {
                 id: None,
                 role: "assistant".into(),
                 content: Vec::new(),
                 end_turn: None,
+                phase: None,
             },
         ];