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/rmcp_client.rs

@@ -394,6 +394,7 @@ async fn stdio_image_responses_are_sanitized_for_text_only_model() -> anyhow::Re
         ModelsResponse {
             models: vec![ModelInfo {
                 slug: text_only_model_slug.to_string(),
+                request_model: None,
                 display_name: "RMCP Text Only".to_string(),
                 description: Some("Test model without image input support".to_string()),
                 default_reasoning_level: None,