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/files.rs

@@ -0,0 +1,58 @@
+use onspring::{OnspringClient, SaveFileRequest};
+
+#[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 record_id: i32 = std::env::var("TEST_RECORD")
+    .expect("TEST_RECORD is required")
+    .parse()
+    .expect("TEST_RECORD must be an integer");
+  let field_id: i32 = std::env::var("TEST_ATTACHMENT_FIELD")
+    .expect("TEST_ATTACHMENT_FIELD is required")
+    .parse()
+    .expect("TEST_ATTACHMENT_FIELD must be an integer");
+  let file_id: i32 = std::env::var("TEST_ATTACHMENT")
+    .expect("TEST_ATTACHMENT is required")
+    .parse()
+    .expect("TEST_ATTACHMENT must be an integer");
+
+  let client = OnspringClient::builder(&api_key)
+    .base_url(&base_url)
+    .build();
+
+  // Get file info
+  let info = client.get_file_info(record_id, field_id, file_id).await?;
+  println!("File info:");
+  println!("  Name: {:?}", info.name);
+  println!("  Content type: {:?}", info.content_type);
+  println!("  Created: {:?}", info.created_date);
+  println!("  Owner: {:?}", info.owner);
+
+  // Download file content
+  let file = client.get_file(record_id, field_id, file_id).await?;
+  println!("\nDownloaded file:");
+  println!("  File name: {:?}", file.file_name);
+  println!("  Content type: {:?}", file.content_type);
+  println!("  Size: {} bytes", file.data.len());
+
+  // Upload a file
+  let request = SaveFileRequest {
+    record_id,
+    field_id,
+    notes: Some("Uploaded from Rust SDK example".to_string()),
+    modified_date: Some(chrono::Utc::now()),
+    file_name: "example-upload.txt".to_string(),
+    file_data: b"Hello from the Rust SDK!".to_vec(),
+    content_type: "text/plain".to_string(),
+  };
+
+  let uploaded = client.upload_file(request).await?;
+  println!("\nUploaded file with ID: {}", uploaded.id);
+
+  // Delete the uploaded file
+  client.delete_file(record_id, field_id, uploaded.id).await?;
+  println!("Deleted file {}", uploaded.id);
+
+  Ok(())
+}