feat: add lists endpoint with save and delete list item methods

Implement SaveListItemRequest and SaveListItemResponse models. Add
save_list_item (create/update via PUT) and delete_list_item methods.
Tests cover creating, updating, deleting list items and not-found error.

src/endpoints/lists.rs

@@ -0,0 +1,25 @@
+use reqwest::Method;
+use uuid::Uuid;
+
+use crate::client::OnspringClient;
+use crate::error::Result;
+use crate::models::{SaveListItemRequest, SaveListItemResponse};
+
+impl OnspringClient {
+    /// Creates or updates a list item in the specified list.
+    pub async fn save_list_item(
+        &self,
+        list_id: i32,
+        request: SaveListItemRequest,
+    ) -> Result<SaveListItemResponse> {
+        let path = format!("/Lists/id/{}/items", list_id);
+        self.request(Method::PUT, &path, &[], Some(&request)).await
+    }
+
+    /// Deletes a list item from the specified list.
+    pub async fn delete_list_item(&self, list_id: i32, item_id: Uuid) -> Result<()> {
+        let path = format!("/Lists/id/{}/itemId/{}", list_id, item_id);
+        self.request_no_content(Method::DELETE, &path, &[], Option::<&()>::None)
+            .await
+    }
+}

src/endpoints/mod.rs

@@ -1,5 +1,6 @@
 mod apps;
 mod fields;
 mod files;
+mod lists;
 mod ping;
 mod records;