feat: add ping endpoint with test infrastructure

Implement ping() method on OnspringClient. Set up wiremock-based test
infrastructure with shared setup helper. Add tests for success and
unauthorized scenarios verifying correct headers are sent.

src/endpoints/mod.rs

@@ -1 +1 @@
-
+mod ping;

src/endpoints/ping.rs

@@ -0,0 +1,12 @@
+use reqwest::Method;
+
+use crate::client::OnspringClient;
+use crate::error::Result;
+
+impl OnspringClient {
+    /// Checks if the Onspring API is reachable.
+    pub async fn ping(&self) -> Result<()> {
+        self.request_no_content(Method::GET, "/Ping", &[], Option::<&()>::None)
+            .await
+    }
+}