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/src/codex_message_processor.rs

@@ -169,6 +169,7 @@ use codex_login::ShutdownHandle;
 use codex_login::run_login_server;
 use codex_protocol::ThreadId;
 use codex_protocol::config_types::ForcedLoginMethod;
+use codex_protocol::config_types::Personality;
 use codex_protocol::items::TurnItem;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::GitInfo as CoreGitInfo;
@@ -1405,6 +1406,7 @@ impl CodexMessageProcessor {
             params.sandbox,
             params.base_instructions,
             params.developer_instructions,
+            params.personality,
         );
 
         let config =
@@ -1518,6 +1520,7 @@ impl CodexMessageProcessor {
         sandbox: Option<SandboxMode>,
         base_instructions: Option<String>,
         developer_instructions: Option<String>,
+        personality: Option<Personality>,
     ) -> ConfigOverrides {
         ConfigOverrides {
             model,
@@ -1529,6 +1532,7 @@ impl CodexMessageProcessor {
             codex_linux_sandbox_exe: self.codex_linux_sandbox_exe.clone(),
             base_instructions,
             developer_instructions,
+            model_personality: personality,
             ..Default::default()
         }
     }
@@ -1836,6 +1840,7 @@ impl CodexMessageProcessor {
             config: request_overrides,
             base_instructions,
             developer_instructions,
+            personality,
         } = params;
 
         let thread_history = if let Some(history) = history {
@@ -1921,6 +1926,7 @@ impl CodexMessageProcessor {
             sandbox,
             base_instructions,
             developer_instructions,
+            personality,
         );
 
         // Derive a Config using the same logic as new conversation, honoring overrides if provided.
@@ -2105,6 +2111,7 @@ impl CodexMessageProcessor {
             sandbox,
             base_instructions,
             developer_instructions,
+            None,
         );
         // Derive a Config using the same logic as new conversation, honoring overrides if provided.
         let config = match derive_config_for_cwd(
@@ -3615,7 +3622,8 @@ impl CodexMessageProcessor {
             || params.model.is_some()
             || params.effort.is_some()
             || params.summary.is_some()
-            || params.collaboration_mode.is_some();
+            || params.collaboration_mode.is_some()
+            || params.personality.is_some();
 
         // If any overrides are provided, update the session turn context first.
         if has_any_overrides {
@@ -3628,7 +3636,7 @@ impl CodexMessageProcessor {
                     effort: params.effort.map(Some),
                     summary: params.summary,
                     collaboration_mode: params.collaboration_mode,
-                    personality: None,
+                    personality: params.personality,
                 })
                 .await;
         }