feat(app-server) Expose personality (#9674)

### Motivation
Exposes a per-thread / per-turn `personality` override in the v2
app-server API so clients can influence model communication style at
thread/turn start. Ensures the override is passed into the session
configuration resolution so it becomes effective for subsequent turns
and headless runners.

### Testing
- [x] Add an integration-style test
`turn_start_accepts_personality_override_v2` in
`codex-rs/app-server/tests/suite/v2/turn_start.rs` that verifies a
`/personality` override results in a developer update message containing
`<personality_spec>` in the outbound model request.

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_6971d646b1c08322a689a54d2649f3fe)

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

@@ -11,18 +11,23 @@ use codex_app_server_protocol::ThreadResumeParams;
 use codex_app_server_protocol::ThreadResumeResponse;
 use codex_app_server_protocol::ThreadStartParams;
 use codex_app_server_protocol::ThreadStartResponse;
+use codex_app_server_protocol::TurnStartParams;
 use codex_app_server_protocol::TurnStatus;
 use codex_app_server_protocol::UserInput;
+use codex_protocol::config_types::Personality;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::user_input::ByteRange;
 use codex_protocol::user_input::TextElement;
+use core_test_support::responses;
+use core_test_support::skip_if_no_network;
 use pretty_assertions::assert_eq;
 use std::path::PathBuf;
 use tempfile::TempDir;
 use tokio::time::timeout;
 
 const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(10);
+const DEFAULT_BASE_INSTRUCTIONS: &str = "You are Codex, based on GPT-5. You are running as a coding agent in the Codex CLI on a user's computer.";
 
 #[tokio::test]
 async fn thread_resume_returns_original_thread() -> Result<()> {
@@ -248,6 +253,91 @@ async fn thread_resume_supports_history_and_overrides() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn thread_resume_accepts_personality_override_v2() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("resp-1"),
+        responses::ev_assistant_message("msg-1", "Done"),
+        responses::ev_completed("resp-1"),
+    ]);
+    let response_mock = responses::mount_sse_once(&server, body).await;
+
+    let codex_home = TempDir::new()?;
+    create_config_toml(codex_home.path(), &server.uri())?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let start_id = mcp
+        .send_thread_start_request(ThreadStartParams {
+            model: Some("gpt-5.2-codex".to_string()),
+            ..Default::default()
+        })
+        .await?;
+    let start_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(start_id)),
+    )
+    .await??;
+    let ThreadStartResponse { thread, .. } = to_response::<ThreadStartResponse>(start_resp)?;
+
+    let resume_id = mcp
+        .send_thread_resume_request(ThreadResumeParams {
+            thread_id: thread.id.clone(),
+            model: Some("gpt-5.2-codex".to_string()),
+            personality: Some(Personality::Friendly),
+            ..Default::default()
+        })
+        .await?;
+    let resume_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(resume_id)),
+    )
+    .await??;
+    let _resume: ThreadResumeResponse = to_response::<ThreadResumeResponse>(resume_resp)?;
+
+    let turn_id = mcp
+        .send_turn_start_request(TurnStartParams {
+            thread_id: thread.id,
+            input: vec![UserInput::Text {
+                text: "Hello".to_string(),
+                text_elements: Vec::new(),
+            }],
+            ..Default::default()
+        })
+        .await?;
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(turn_id)),
+    )
+    .await??;
+
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_notification_message("turn/completed"),
+    )
+    .await??;
+
+    let request = response_mock.single_request();
+    let developer_texts = request.message_input_texts("developer");
+    assert!(
+        !developer_texts
+            .iter()
+            .any(|text| text.contains("<personality_spec>")),
+        "did not expect a personality update message in developer input, got {developer_texts:?}"
+    );
+    let instructions_text = request.instructions_text();
+    assert!(
+        instructions_text.contains(DEFAULT_BASE_INSTRUCTIONS),
+        "expected default base instructions from history, got {instructions_text:?}"
+    );
+
+    Ok(())
+}
+
 // Helper to create a config.toml pointing at the mock model server.
 fn create_config_toml(codex_home: &std::path::Path, server_uri: &str) -> std::io::Result<()> {
     let config_toml = codex_home.join("config.toml");
@@ -261,6 +351,9 @@ sandbox_mode = "read-only"
 
 model_provider = "mock_provider"
 
+[features]
+remote_models = false
+
 [model_providers.mock_provider]
 name = "Mock provider for test"
 base_url = "{server_uri}/v1"