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/unified_exec.rs

@@ -210,6 +210,7 @@ async fn unified_exec_intercepts_apply_patch_exec_command() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -338,6 +339,7 @@ async fn unified_exec_emits_exec_command_begin_event() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -415,6 +417,7 @@ async fn unified_exec_resolves_relative_workdir() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -495,6 +498,7 @@ async fn unified_exec_respects_workdir_override() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -587,6 +591,7 @@ async fn unified_exec_emits_exec_command_end_event() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -661,6 +666,7 @@ async fn unified_exec_emits_output_delta_for_exec_command() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -736,6 +742,7 @@ async fn unified_exec_full_lifecycle_with_background_end_event() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -865,6 +872,7 @@ async fn unified_exec_emits_terminal_interaction_for_write_stdin() -> Result<()>
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -1001,6 +1009,7 @@ async fn unified_exec_terminal_interaction_captures_delayed_output() -> Result<(
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -1160,6 +1169,7 @@ async fn unified_exec_emits_one_begin_and_one_end_event() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -1257,6 +1267,7 @@ async fn exec_command_reports_chunk_and_exit_metadata() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -1374,6 +1385,7 @@ async fn unified_exec_defaults_to_pipe() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -1463,6 +1475,7 @@ async fn unified_exec_can_enable_tty() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -1543,6 +1556,7 @@ async fn unified_exec_respects_early_exit_notifications() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -1673,6 +1687,7 @@ async fn write_stdin_returns_exit_metadata_and_clears_session() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -1840,6 +1855,7 @@ async fn unified_exec_emits_end_event_when_session_dies_via_stdin() -> Result<()
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -1916,6 +1932,7 @@ async fn unified_exec_closes_long_running_session_at_turn_end() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -2037,6 +2054,7 @@ async fn unified_exec_reuses_session_via_stdin() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -2171,6 +2189,7 @@ PY
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
     // This is a worst case scenario for the truncate logic.
@@ -2284,6 +2303,7 @@ async fn unified_exec_timeout_and_followup_poll() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -2379,6 +2399,7 @@ PY
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -2460,6 +2481,7 @@ async fn unified_exec_runs_under_sandbox() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -2563,6 +2585,7 @@ async fn unified_exec_python_prompt_under_seatbelt() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -2657,6 +2680,7 @@ async fn unified_exec_runs_on_all_platforms() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -2791,6 +2815,7 @@ async fn unified_exec_prunes_exited_sessions_first() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;