backend-client: add get_config_requirements_file (#10001)

Adds getting config requirement to backend-client.

I made a slash command to test it (not included in this PR):
<img width="726" height="330" alt="Screenshot 2026-01-27 at 15 20 41"
src="https://github.com/user-attachments/assets/97222e7c-5078-485a-a5b2-a6630313901e"
/>

codex-rs/codex-backend-openapi-models/src/models/mod.rs

@@ -3,6 +3,10 @@
 // Currently export only the types referenced by the workspace
 // The process for this will change
 
+// Config
+pub mod config_file_response;
+pub use self::config_file_response::ConfigFileResponse;
+
 // Cloud Tasks
 pub mod code_task_details_response;
 pub use self::code_task_details_response::CodeTaskDetailsResponse;