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/fields.rs

@@ -0,0 +1,56 @@
+use onspring::{OnspringClient, PagingRequest};
+
+#[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 app_id: i32 = std::env::var("TEST_APP_ID")
+    .expect("TEST_APP_ID is required")
+    .parse()
+    .expect("TEST_APP_ID must be an integer");
+
+  let client = OnspringClient::builder(&api_key)
+    .base_url(&base_url)
+    .build();
+
+  // List fields for an app
+  let paging = PagingRequest {
+    page_number: 1,
+    page_size: 10,
+  };
+  let response = client.list_fields(app_id, Some(paging)).await?;
+  println!("Total fields: {:?}", response.total_records);
+
+  if let Some(items) = &response.items {
+    for field in items {
+      println!(
+        "  Field: {} (id: {}, type: {:?})",
+        field.name.as_deref().unwrap_or("N/A"),
+        field.id,
+        field.field_type.as_deref().unwrap_or("N/A"),
+      );
+    }
+
+    // Get a single field
+    if let Some(first) = items.first() {
+      let field = client.get_field(first.id).await?;
+      println!(
+        "\nFetched field: {} (id: {})",
+        field.name.as_deref().unwrap_or("N/A"),
+        field.id
+      );
+    }
+
+    // Batch get fields
+    let ids: Vec<i32> = items.iter().take(3).map(|f| f.id).collect();
+    if !ids.is_empty() {
+      let batch = client.batch_get_fields(&ids).await?;
+      println!(
+        "\nBatch fetched {} fields",
+        batch.items.map(|i| i.len()).unwrap_or(0)
+      );
+    }
+  }
+
+  Ok(())
+}