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/common/mod.rs

@@ -0,0 +1,10 @@
+use onspring::OnspringClient;
+use wiremock::MockServer;
+
+pub async fn setup() -> (MockServer, OnspringClient) {
+    let mock_server = MockServer::start().await;
+    let client = OnspringClient::builder("test-api-key")
+        .base_url(mock_server.uri())
+        .build();
+    (mock_server, client)
+}