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.

tests/test_ping.rs

@@ -0,0 +1,40 @@
+mod common;
+
+use wiremock::matchers::{header, method, path};
+use wiremock::{Mock, ResponseTemplate};
+
+#[tokio::test]
+async fn test_ping_success() {
+    let (mock_server, client) = common::setup().await;
+
+    Mock::given(method("GET"))
+        .and(path("/Ping"))
+        .and(header("x-apikey", "test-api-key"))
+        .and(header("x-api-version", "2"))
+        .respond_with(ResponseTemplate::new(200))
+        .mount(&mock_server)
+        .await;
+
+    let result = client.ping().await;
+    assert!(result.is_ok());
+}
+
+#[tokio::test]
+async fn test_ping_unauthorized() {
+    let (mock_server, client) = common::setup().await;
+
+    Mock::given(method("GET"))
+        .and(path("/Ping"))
+        .respond_with(ResponseTemplate::new(401))
+        .mount(&mock_server)
+        .await;
+
+    let result = client.ping().await;
+    assert!(result.is_err());
+
+    if let Err(onspring::OnspringError::Api { status_code, .. }) = result {
+        assert_eq!(status_code, 401);
+    } else {
+        panic!("Expected Api error");
+    }
+}