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/README.md

@@ -114,6 +114,7 @@ Start a fresh thread when you need a new Codex conversation.
     "cwd": "/Users/me/project",
     "approvalPolicy": "never",
     "sandbox": "workspaceWrite",
+    "personality": "friendly"
 } }
 { "id": 10, "result": {
     "thread": {
@@ -126,10 +127,13 @@ Start a fresh thread when you need a new Codex conversation.
 { "method": "thread/started", "params": { "thread": { … } } }
 ```
 
-To continue a stored session, call `thread/resume` with the `thread.id` you previously recorded. The response shape matches `thread/start`, and no additional notifications are emitted:
+To continue a stored session, call `thread/resume` with the `thread.id` you previously recorded. The response shape matches `thread/start`, and no additional notifications are emitted. You can also pass the same configuration overrides supported by `thread/start`, such as `personality`:
 
 ```json
-{ "method": "thread/resume", "id": 11, "params": { "threadId": "thr_123" } }
+{ "method": "thread/resume", "id": 11, "params": {
+    "threadId": "thr_123",
+    "personality": "friendly"
+} }
 { "id": 11, "result": { "thread": { "id": "thr_123", … } } }
 ```
 
@@ -231,6 +235,7 @@ You can optionally specify config overrides on the new turn. If specified, these
     "model": "gpt-5.1-codex",
     "effort": "medium",
     "summary": "concise",
+    "personality": "friendly",
     // Optional JSON Schema to constrain the final assistant message for this turn.
     "outputSchema": {
         "type": "object",