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/config_file_response.rs

@@ -0,0 +1,40 @@
+/*
+ * codex-backend
+ *
+ * codex-backend
+ *
+ * The version of the OpenAPI document: 0.0.1
+ *
+ * Generated by: https://openapi-generator.tech
+ */
+
+use serde::Deserialize;
+use serde::Serialize;
+
+#[derive(Clone, Default, Debug, PartialEq, Serialize, Deserialize)]
+pub struct ConfigFileResponse {
+    #[serde(rename = "contents", skip_serializing_if = "Option::is_none")]
+    pub contents: Option<String>,
+    #[serde(rename = "sha256", skip_serializing_if = "Option::is_none")]
+    pub sha256: Option<String>,
+    #[serde(rename = "updated_at", skip_serializing_if = "Option::is_none")]
+    pub updated_at: Option<String>,
+    #[serde(rename = "updated_by_user_id", skip_serializing_if = "Option::is_none")]
+    pub updated_by_user_id: Option<String>,
+}
+
+impl ConfigFileResponse {
+    pub fn new(
+        contents: Option<String>,
+        sha256: Option<String>,
+        updated_at: Option<String>,
+        updated_by_user_id: Option<String>,
+    ) -> ConfigFileResponse {
+        ConfigFileResponse {
+            contents,
+            sha256,
+            updated_at,
+            updated_by_user_id,
+        }
+    }
+}

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;