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/core/tests/suite/model_switching.rs

@@ -68,6 +68,7 @@ fn test_model_info(
 ) -> ModelInfo {
     ModelInfo {
         slug: slug.to_string(),
+        request_model: None,
         display_name: display_name.to_string(),
         description: Some(description.to_string()),
         default_reasoning_level: Some(ReasoningEffort::Medium),
@@ -884,6 +885,7 @@ async fn model_switch_to_smaller_model_updates_token_context_window() -> Result<
 
     let base_model = ModelInfo {
         slug: large_model_slug.to_string(),
+        request_model: None,
         display_name: "Larger Model".to_string(),
         description: Some("larger context window model".to_string()),
         default_reasoning_level: Some(ReasoningEffort::Medium),