Reject token-budget history notes for unsupported starting models (#44883)

## What changed

After resolving startup configuration and model defaults, reject `features.token_budget.use_history_notes_extension` when the starting model lacks `supports_experimental_context`. Return an error directing users to disable the option or select a compatible model.

## Testing

Add startup coverage for explicit configuration and model defaults, verifying rejection for unsupported models and successful activation for supported models and standalone token budgets. Update history-notes test fixtures to declare experimental context support.

GitOrigin-RevId: abf1a024efc5acf97cfc858fbb93821363769dc0

codex-rs/ext/history-notes/tests/image_model_input.rs

@@ -64,6 +64,9 @@ async fn history_images_reach_the_next_model_request() -> Result<(), Box<dyn std
     );
     let test = test_codex()
         .with_auth(auth)
+        .with_model_info_override("gpt-5.5", |model_info| {
+            model_info.supports_experimental_context = true;
+        })
         .with_extensions(Arc::new(extensions.build()))
         .with_config(|config| {
             config.model_provider.name = "OpenAI".to_string();