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/core/tests/suite/remote_models.rs

@@ -79,7 +79,7 @@ async fn remote_models_remote_model_uses_unified_exec() -> Result<()> {
         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,
@@ -316,7 +316,7 @@ async fn remote_models_apply_remote_base_instructions() -> Result<()> {
         priority: 1,
         upgrade: None,
         base_instructions: remote_base.to_string(),
-        model_instructions_template: None,
+        model_messages: None,
         supports_reasoning_summaries: false,
         support_verbosity: false,
         default_verbosity: None,
@@ -790,7 +790,7 @@ fn test_remote_model_with_policy(
         priority,
         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,