feat(core) update Personality on turn (#9644)

## Summary
Support updating Personality mid-Thread via UserTurn/OverwriteTurn. This
is explicitly unused by the clients so far, to simplify PRs - app-server
and tui implementations will be follow-ups.

## Testing
- [x] added integration tests

codex-rs/protocol/src/models.rs

@@ -225,6 +225,13 @@ impl DeveloperInstructions {
         Self { text }
     }
 
+    pub fn personality_spec_message(spec: String) -> Self {
+        let message = format!(
+            "<personality_spec> The user has requested a new communication style. Future messages should adhere to the following personality: \n{spec} </personality_spec>"
+        );
+        DeveloperInstructions::new(message)
+    }
+
     pub fn from_policy(
         sandbox_policy: &SandboxPolicy,
         approval_policy: AskForApproval,

codex-rs/protocol/src/protocol.rs

@@ -14,6 +14,7 @@ use std::time::Duration;
 use crate::ThreadId;
 use crate::approvals::ElicitationRequestEvent;
 use crate::config_types::CollaborationMode;
+use crate::config_types::Personality;
 use crate::config_types::ReasoningSummary as ReasoningSummaryConfig;
 use crate::custom_prompts::CustomPrompt;
 use crate::items::TurnItem;
@@ -129,6 +130,10 @@ pub enum Op {
         /// Takes precedence over model, effort, and developer instructions if set.
         #[serde(skip_serializing_if = "Option::is_none")]
         collaboration_mode: Option<CollaborationMode>,
+
+        /// Optional personality override for this turn.
+        #[serde(skip_serializing_if = "Option::is_none")]
+        personality: Option<Personality>,
     },
 
     /// Override parts of the persistent turn context for subsequent turns.
@@ -170,6 +175,10 @@ pub enum Op {
         /// Takes precedence over model, effort, and developer instructions if set.
         #[serde(skip_serializing_if = "Option::is_none")]
         collaboration_mode: Option<CollaborationMode>,
+
+        /// Updated personality preference.
+        #[serde(skip_serializing_if = "Option::is_none")]
+        personality: Option<Personality>,
     },
 
     /// Approve a command execution
@@ -1618,6 +1627,8 @@ pub struct TurnContextItem {
     pub approval_policy: AskForApproval,
     pub sandbox_policy: SandboxPolicy,
     pub model: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub personality: Option<Personality>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub collaboration_mode: Option<CollaborationMode>,
     #[serde(skip_serializing_if = "Option::is_none")]