feat(core): support custom model aliases in config.toml

Preserve custom model alias support on the current origin/main base and collapse the branch back to a single commit for easier future restacks.

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

@@ -15,6 +15,7 @@ use std::path::Path;
 fn preset_to_info(preset: &ModelPreset, priority: i32) -> ModelInfo {
     ModelInfo {
         slug: preset.id.clone(),
+        request_model: None,
         display_name: preset.display_name.clone(),
         description: Some(preset.description.clone()),
         default_reasoning_level: Some(preset.default_reasoning_effort),