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/exec/src/lib.rs

@@ -211,6 +211,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         codex_linux_sandbox_exe,
         base_instructions: None,
         developer_instructions: None,
+        model_personality: None,
         compact_prompt: None,
         include_apply_patch_tool: None,
         show_raw_agent_reasoning: oss.then_some(true),