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/rmcp_client.rs

@@ -119,6 +119,7 @@ async fn stdio_server_round_trip() -> anyhow::Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -258,6 +259,7 @@ async fn stdio_image_responses_round_trip() -> anyhow::Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -455,6 +457,7 @@ async fn stdio_image_completions_round_trip() -> anyhow::Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -600,6 +603,7 @@ async fn stdio_server_propagates_whitelisted_env_vars() -> anyhow::Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -756,6 +760,7 @@ async fn streamable_http_tool_call_round_trip() -> anyhow::Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -944,6 +949,7 @@ async fn streamable_http_with_oauth_round_trip() -> anyhow::Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;