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_common/mod.rs

@@ -0,0 +1,54 @@
+use onspring::OnspringClient;
+use std::collections::HashMap;
+
+pub fn required_env(name: &str) -> String {
+  std::env::var(name).unwrap_or_else(|_| panic!("{} is not defined", name))
+}
+
+pub fn required_env_i32(name: &str) -> i32 {
+  required_env(name)
+    .parse()
+    .unwrap_or_else(|_| panic!("{} is not a valid integer", name))
+}
+
+pub fn required_env_csv_i32(name: &str) -> Vec<i32> {
+  required_env(name)
+    .split(',')
+    .map(|s| {
+      s.trim()
+        .parse()
+        .unwrap_or_else(|_| panic!("{} contains non-integer values", name))
+    })
+    .collect()
+}
+
+pub fn build_client() -> OnspringClient {
+  let base_url = required_env("API_BASE_URL");
+  let api_key = required_env("SANDBOX_API_KEY");
+  OnspringClient::builder(&api_key)
+    .base_url(&base_url)
+    .build()
+}
+
+pub fn build_client_with_key(api_key: &str) -> OnspringClient {
+  let base_url = required_env("API_BASE_URL");
+  OnspringClient::builder(api_key).base_url(&base_url).build()
+}
+
+pub async fn add_record() -> i32 {
+  let client = build_client();
+  let app_id = required_env_i32("TEST_SURVEY_ID");
+  let field_id = required_env("TEST_TEXT_FIELD");
+
+  let mut fields = HashMap::new();
+  fields.insert(field_id, serde_json::json!("test"));
+
+  let request = onspring::SaveRecordRequest {
+    app_id,
+    record_id: None,
+    fields,
+  };
+
+  let response = client.save_record(request).await.unwrap();
+  response.id
+}