feat: add apps endpoint with list, get, and batch-get methods

Implement App model and three endpoint methods: list_apps (paginated),
get_app (by ID), and batch_get_apps (up to 100 by IDs). Add
comprehensive tests for success, pagination, not-found, and batch
scenarios.

src/endpoints/apps.rs

@@ -0,0 +1,32 @@
+use reqwest::Method;
+
+use crate::client::OnspringClient;
+use crate::error::Result;
+use crate::models::{App, CollectionResponse, PagedResponse, PagingRequest};
+
+impl OnspringClient {
+    /// Gets all apps for the current client, with optional pagination.
+    pub async fn list_apps(&self, paging: Option<PagingRequest>) -> Result<PagedResponse<App>> {
+        let mut query = Vec::new();
+        if let Some(p) = paging {
+            query.push(("PageNumber", p.page_number.to_string()));
+            query.push(("PageSize", p.page_size.to_string()));
+        }
+        let query_refs: Vec<(&str, String)> = query.iter().map(|(k, v)| (*k, v.clone())).collect();
+        self.request(Method::GET, "/Apps", &query_refs, Option::<&()>::None)
+            .await
+    }
+
+    /// Gets an app by its identifier.
+    pub async fn get_app(&self, app_id: i32) -> Result<App> {
+        let path = format!("/Apps/id/{}", app_id);
+        self.request(Method::GET, &path, &[], Option::<&()>::None)
+            .await
+    }
+
+    /// Gets up to 100 apps by their identifiers.
+    pub async fn batch_get_apps(&self, ids: &[i32]) -> Result<CollectionResponse<App>> {
+        self.request(Method::POST, "/Apps/batch-get", &[], Some(&ids))
+            .await
+    }
+}

src/endpoints/mod.rs

@@ -1 +1,2 @@
+mod apps;
 mod ping;