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/tests/suite/tool_harness.rs

@@ -91,6 +91,7 @@ async fn shell_tool_executes_command_and_streams_output() -> anyhow::Result<()>
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -159,6 +160,7 @@ async fn update_plan_tool_emits_plan_update_event() -> anyhow::Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -237,6 +239,7 @@ async fn update_plan_tool_rejects_malformed_payload() -> anyhow::Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -327,6 +330,7 @@ async fn apply_patch_tool_executes_and_emits_patch_events() -> anyhow::Result<()
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -425,6 +429,7 @@ async fn apply_patch_reports_parse_diagnostics() -> anyhow::Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;