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;

src/models/list.rs

@@ -0,0 +1,22 @@
+use serde::{Deserialize, Serialize};
+use uuid::Uuid;
+
+/// Request to create or update a list item.
+#[derive(Debug, Clone, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SaveListItemRequest {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub id: Option<Uuid>,
+    pub name: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub numeric_value: Option<f64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub color: Option<String>,
+}
+
+/// Response from saving a list item.
+#[derive(Debug, Clone, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SaveListItemResponse {
+    pub id: Uuid,
+}

src/models/mod.rs

@@ -2,6 +2,7 @@ mod app;
 mod enums;
 mod field;
 pub mod file;
+pub mod list;
 mod paging;
 pub mod record;
 
@@ -9,5 +10,6 @@ pub use app::*;
 pub use enums::*;
 pub use field::*;
 pub use file::*;
+pub use list::*;
 pub use paging::*;
 pub use record::*;