feat: add integration tests and runnable examples

Add 91 integration tests (all #[ignore]) mirroring the JavaScript SDK's
integration test suite. Tests cover all endpoints: ping, apps, fields,
records, files, lists, and reports with success, auth, access, and
not-found scenarios.

Add 7 runnable examples (cargo run --example <name>) for quick manual
smoke testing against a real Onspring instance.

Both use env vars (API_BASE_URL, SANDBOX_API_KEY, TEST_*) matching the
JavaScript SDK setup.

Also rename crate to onspring-api-sdk-rust with [lib] name = "onspring"
so imports remain `use onspring::*`.

tests/integration_ping.rs

@@ -0,0 +1,18 @@
+mod integration_common;
+
+use integration_common::{build_client, build_client_with_key};
+
+#[tokio::test]
+#[ignore]
+async fn ping_should_succeed() {
+  let client = build_client();
+  client.ping().await.unwrap();
+}
+
+#[tokio::test]
+#[ignore]
+async fn ping_should_fail_with_invalid_api_key() {
+  let client = build_client_with_key("invalid");
+  let result = client.ping().await;
+  assert!(result.is_err());
+}