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-protocol/src/protocol/v2.rs

@@ -6,6 +6,7 @@ use codex_protocol::account::PlanType;
 use codex_protocol::approvals::ExecPolicyAmendment as CoreExecPolicyAmendment;
 use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::ForcedLoginMethod;
+use codex_protocol::config_types::Personality;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::SandboxMode as CoreSandboxMode;
 use codex_protocol::config_types::Verbosity;
@@ -1079,6 +1080,7 @@ pub struct ThreadStartParams {
     pub config: Option<HashMap<String, JsonValue>>,
     pub base_instructions: Option<String>,
     pub developer_instructions: Option<String>,
+    pub personality: Option<Personality>,
     /// If true, opt into emitting raw response items on the event stream.
     ///
     /// This is for internal use only (e.g. Codex Cloud).
@@ -1133,6 +1135,7 @@ pub struct ThreadResumeParams {
     pub config: Option<HashMap<String, serde_json::Value>>,
     pub base_instructions: Option<String>,
     pub developer_instructions: Option<String>,
+    pub personality: Option<Personality>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
@@ -1608,6 +1611,8 @@ pub struct TurnStartParams {
     pub effort: Option<ReasoningEffort>,
     /// Override the reasoning summary for this turn and subsequent turns.
     pub summary: Option<ReasoningSummary>,
+    /// Override the personality for this turn and subsequent turns.
+    pub personality: Option<Personality>,
     /// Optional JSON Schema used to constrain the final assistant message for this turn.
     pub output_schema: Option<JsonValue>,