chore(personality) new schema with fallbacks (#10147)

## Summary
Let's dial in this api contract in a bit more with more robust fallback
behavior when model_instructions_template is false.

Switches to a more explicit template / variables structure, with more
fallbacks.

## Testing
- [x] Adding unit tests
- [x] Tested locally

codex-rs/codex-api/tests/models_integration.rs

@@ -77,7 +77,7 @@ async fn models_client_hits_models_endpoint() {
             priority: 1,
             upgrade: None,
             base_instructions: "base instructions".to_string(),
-            model_instructions_template: None,
+            model_messages: None,
             supports_reasoning_summaries: false,
             support_verbosity: false,
             default_verbosity: None,