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/app-server/tests/common/models_cache.rs

@@ -27,7 +27,7 @@ fn preset_to_info(preset: &ModelPreset, priority: i32) -> ModelInfo {
         priority,
         upgrade: preset.upgrade.as_ref().map(|u| u.into()),
         base_instructions: "base instructions".to_string(),
-        model_instructions_template: None,
+        model_messages: None,
         supports_reasoning_summaries: false,
         support_verbosity: false,
         default_verbosity: None,