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::*`.

examples/ping.rs

@@ -0,0 +1,16 @@
+use onspring::OnspringClient;
+
+#[tokio::main]
+async fn main() -> onspring::Result<()> {
+  let base_url = std::env::var("API_BASE_URL").expect("API_BASE_URL is required");
+  let api_key = std::env::var("SANDBOX_API_KEY").expect("SANDBOX_API_KEY is required");
+
+  let client = OnspringClient::builder(&api_key)
+    .base_url(&base_url)
+    .build();
+
+  client.ping().await?;
+  println!("Connected to the Onspring API successfully.");
+
+  Ok(())
+}