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/core/src/session/mod.rs

@@ -707,6 +707,16 @@ impl Session {
                 &model_info,
             )?;
             token_budget::apply_model_defaults(Arc::make_mut(&mut config), &model_info);
+            if config
+                .token_budget
+                .as_ref()
+                .is_some_and(|token_budget| token_budget.use_history_notes_extension)
+                && !model_info.supports_experimental_context
+            {
+                return Err(CodexErr::InvalidRequest(format!(
+                    "features.token_budget.use_history_notes_extension is not supported by model `{model}`; disable it or select a model that supports experimental context"
+                )));
+            }
         }
         let configured_config = Arc::clone(&config);
         let multi_agent_version = config.multi_agent_version_override().or_else(|| {