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_fields.rs

@@ -0,0 +1,126 @@
+mod integration_common;
+
+use integration_common::*;
+use onspring::PagingRequest;
+
+#[tokio::test]
+#[ignore]
+async fn get_field_should_return_field() {
+  let client = build_client();
+  let field_id = required_env_i32("TEST_FIELD_ID");
+  let field = client.get_field(field_id).await.unwrap();
+
+  assert_eq!(field.id, field_id);
+  assert!(field.name.is_some());
+  assert!(field.app_id > 0);
+}
+
+#[tokio::test]
+#[ignore]
+async fn get_field_should_fail_with_invalid_api_key() {
+  let client = build_client_with_key("invalid");
+  let result = client.get_field(1).await;
+  assert!(result.is_err());
+}
+
+#[tokio::test]
+#[ignore]
+async fn get_field_should_fail_when_no_access() {
+  let client = build_client();
+  let field_id = required_env_i32("TEST_FIELD_ID_NO_ACCESS");
+  let result = client.get_field(field_id).await;
+  assert!(result.is_err());
+}
+
+#[tokio::test]
+#[ignore]
+async fn get_field_should_fail_when_not_found() {
+  let client = build_client();
+  let result = client.get_field(0).await;
+  assert!(result.is_err());
+}
+
+#[tokio::test]
+#[ignore]
+async fn list_fields_should_return_fields() {
+  let client = build_client();
+  let app_id = required_env_i32("TEST_SURVEY_ID");
+  let response = client.list_fields(app_id, None).await.unwrap();
+
+  assert!(response.total_records.unwrap() > 0);
+  let items = response.items.unwrap();
+  assert!(!items.is_empty());
+
+  for field in &items {
+    assert!(field.id > 0);
+    assert!(field.name.is_some());
+    assert!(field.app_id > 0);
+  }
+}
+
+#[tokio::test]
+#[ignore]
+async fn list_fields_should_respect_paging() {
+  let client = build_client();
+  let app_id = required_env_i32("TEST_SURVEY_ID");
+  let paging = PagingRequest {
+    page_number: 1,
+    page_size: 1,
+  };
+  let response = client.list_fields(app_id, Some(paging)).await.unwrap();
+
+  assert_eq!(response.page_number.unwrap(), 1);
+  assert_eq!(response.page_size.unwrap(), 1);
+  assert_eq!(response.items.unwrap().len(), 1);
+}
+
+#[tokio::test]
+#[ignore]
+async fn list_fields_should_fail_with_invalid_api_key() {
+  let client = build_client_with_key("invalid");
+  let result = client.list_fields(1, None).await;
+  assert!(result.is_err());
+}
+
+#[tokio::test]
+#[ignore]
+async fn list_fields_should_fail_when_no_access() {
+  let client = build_client();
+  let app_id = required_env_i32("TEST_APP_ID_NO_ACCESS");
+  let result = client.list_fields(app_id, None).await;
+  assert!(result.is_err());
+}
+
+#[tokio::test]
+#[ignore]
+async fn batch_get_fields_should_return_fields() {
+  let client = build_client();
+  let field_ids = required_env_csv_i32("TEST_FIELD_IDS");
+  let response = client.batch_get_fields(&field_ids).await.unwrap();
+
+  let items = response.items.unwrap();
+  assert_eq!(items.len(), field_ids.len());
+
+  for field in &items {
+    assert!(field.id > 0);
+    assert!(field.name.is_some());
+    assert!(field.app_id > 0);
+  }
+}
+
+#[tokio::test]
+#[ignore]
+async fn batch_get_fields_should_fail_with_invalid_api_key() {
+  let client = build_client_with_key("invalid");
+  let result = client.batch_get_fields(&[1, 2, 3]).await;
+  assert!(result.is_err());
+}
+
+#[tokio::test]
+#[ignore]
+async fn batch_get_fields_should_fail_when_no_access() {
+  let client = build_client();
+  let field_ids = required_env_csv_i32("TEST_FIELD_IDS_NO_ACCESS");
+  let result = client.batch_get_fields(&field_ids).await;
+  assert!(result.is_err());
+}