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/view_image.rs

@@ -1335,6 +1335,7 @@ async fn view_image_tool_returns_unsupported_message_for_text_only_model() -> an
     let model_slug = "text-only-view-image-test-model";
     let text_only_model = ModelInfo {
         slug: model_slug.to_string(),
+        request_model: None,
         display_name: "Text-only view_image test model".to_string(),
         description: Some("Remote model for view_image unsupported-path coverage".to_string()),
         default_reasoning_level: Some(ReasoningEffort::Medium),