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/codex-api/tests/models_integration.rs

@@ -54,6 +54,7 @@ async fn models_client_hits_models_endpoint() {
     let response = ModelsResponse {
         models: vec![ModelInfo {
             slug: "gpt-test".to_string(),
+            request_model: None,
             display_name: "gpt-test".to_string(),
             description: Some("desc".to_string()),
             default_reasoning_level: Some(ReasoningEffort::Medium),