fix(core) switching model appends model instructions (#10651)

## Summary
When switching models, we should append the instructions of the new
model to the conversation as a developer message.

## Test
- [x] Adds a unit test

codex-rs/core/tests/suite/personality.rs

@@ -882,7 +882,7 @@ async fn user_turn_personality_remote_model_template_includes_update_message() -
             cwd: test.cwd_path().to_path_buf(),
             approval_policy: AskForApproval::Never,
             sandbox_policy: SandboxPolicy::ReadOnly,
-            model: test.session_configured.model.clone(),
+            model: remote_slug.to_string(),
             effort: test.config.model_reasoning_effort,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
@@ -898,7 +898,7 @@ async fn user_turn_personality_remote_model_template_includes_update_message() -
             approval_policy: None,
             sandbox_policy: None,
             windows_sandbox_level: None,
-            model: Some(remote_slug.to_string()),
+            model: None,
             effort: None,
             summary: None,
             collaboration_mode: None,