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/core/src/context_manager/history.rs

@@ -86,7 +86,9 @@ impl ContextManager {
     // This is a coarse lower bound, not a tokenizer-accurate count.
     pub(crate) fn estimate_token_count(&self, turn_context: &TurnContext) -> Option<i64> {
         let model_info = turn_context.client.get_model_info();
-        let personality = turn_context.client.config().model_personality;
+        let personality = turn_context
+            .personality
+            .or(turn_context.client.config().model_personality);
         let base_instructions = model_info.get_model_instructions(personality);
         let base_tokens = i64::try_from(approx_token_count(&base_instructions)).unwrap_or(i64::MAX);