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/lists.rs

@@ -0,0 +1,45 @@
+use onspring::{OnspringClient, SaveListItemRequest};
+
+#[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 list_id: i32 = std::env::var("TEST_LIST_ID")
+    .expect("TEST_LIST_ID is required")
+    .parse()
+    .expect("TEST_LIST_ID must be an integer");
+
+  let client = OnspringClient::builder(&api_key)
+    .base_url(&base_url)
+    .build();
+
+  // Add a list item
+  let name = format!("rust_sdk_example_{}", chrono::Utc::now().timestamp_millis());
+  let request = SaveListItemRequest {
+    id: None,
+    name: name.clone(),
+    numeric_value: Some(42.0),
+    color: Some("#FF5733".to_string()),
+  };
+
+  let response = client.save_list_item(list_id, request).await?;
+  println!("Added list item '{}' with ID: {}", name, response.id);
+
+  // Update the list item
+  let update_name = format!("updated_{}", name);
+  let update_request = SaveListItemRequest {
+    id: Some(response.id),
+    name: update_name.clone(),
+    numeric_value: Some(99.0),
+    color: Some("#00FF00".to_string()),
+  };
+
+  client.save_list_item(list_id, update_request).await?;
+  println!("Updated list item to '{}'", update_name);
+
+  // Delete the list item
+  client.delete_list_item(list_id, response.id).await?;
+  println!("Deleted list item {}", response.id);
+
+  Ok(())
+}