feat: add files endpoint with info, download, upload, and delete

Implement FileInfo, FileResponse, SaveFileRequest, and
CreatedWithIdResponse models. Add get_file_info, get_file (binary
download), upload_file (multipart), and delete_file methods. Tests
cover metadata retrieval, binary content download with headers, file
upload, deletion, and not-found error with message extraction.

tests/test_files.rs

@@ -0,0 +1,116 @@
+mod common;
+
+use wiremock::matchers::{method, path};
+use wiremock::{Mock, ResponseTemplate};
+
+#[tokio::test]
+async fn test_get_file_info() {
+    let (mock_server, client) = common::setup().await;
+
+    Mock::given(method("GET"))
+        .and(path("/Files/recordId/1/fieldId/2/fileId/3"))
+        .respond_with(ResponseTemplate::new(200).set_body_json(serde_json::json!({
+            "type": "Attachment",
+            "contentType": "application/pdf",
+            "name": "document.pdf",
+            "createdDate": "2024-01-15T10:30:00Z",
+            "modifiedDate": "2024-01-15T10:30:00Z",
+            "owner": "admin",
+            "notes": "Important doc",
+            "fileHref": "/Files/recordId/1/fieldId/2/fileId/3/file"
+        })))
+        .mount(&mock_server)
+        .await;
+
+    let info = client.get_file_info(1, 2, 3).await.unwrap();
+    assert_eq!(info.name.as_deref(), Some("document.pdf"));
+    assert_eq!(info.content_type.as_deref(), Some("application/pdf"));
+    assert_eq!(info.owner.as_deref(), Some("admin"));
+}
+
+#[tokio::test]
+async fn test_get_file_content() {
+    let (mock_server, client) = common::setup().await;
+
+    let file_bytes = b"hello file content";
+    Mock::given(method("GET"))
+        .and(path("/Files/recordId/1/fieldId/2/fileId/3/file"))
+        .respond_with(
+            ResponseTemplate::new(200)
+                .set_body_bytes(file_bytes.to_vec())
+                .insert_header("content-type", "application/pdf")
+                .insert_header("content-disposition", "attachment; filename=\"test.pdf\""),
+        )
+        .mount(&mock_server)
+        .await;
+
+    let response = client.get_file(1, 2, 3).await.unwrap();
+    assert_eq!(response.data.as_ref(), file_bytes);
+    assert_eq!(response.content_type.as_deref(), Some("application/pdf"));
+    assert_eq!(response.file_name.as_deref(), Some("test.pdf"));
+}
+
+#[tokio::test]
+async fn test_upload_file() {
+    let (mock_server, client) = common::setup().await;
+
+    Mock::given(method("POST"))
+        .and(path("/Files"))
+        .respond_with(ResponseTemplate::new(201).set_body_json(serde_json::json!({
+            "id": 42
+        })))
+        .mount(&mock_server)
+        .await;
+
+    let request = onspring::models::file::SaveFileRequest {
+        record_id: 1,
+        field_id: 2,
+        notes: Some("test notes".to_string()),
+        modified_date: None,
+        file_name: "test.txt".to_string(),
+        file_data: b"file content".to_vec(),
+        content_type: "text/plain".to_string(),
+    };
+    let result = client.upload_file(request).await.unwrap();
+    assert_eq!(result.id, 42);
+}
+
+#[tokio::test]
+async fn test_delete_file() {
+    let (mock_server, client) = common::setup().await;
+
+    Mock::given(method("DELETE"))
+        .and(path("/Files/recordId/1/fieldId/2/fileId/3"))
+        .respond_with(ResponseTemplate::new(204))
+        .mount(&mock_server)
+        .await;
+
+    let result = client.delete_file(1, 2, 3).await;
+    assert!(result.is_ok());
+}
+
+#[tokio::test]
+async fn test_get_file_info_not_found() {
+    let (mock_server, client) = common::setup().await;
+
+    Mock::given(method("GET"))
+        .and(path("/Files/recordId/1/fieldId/2/fileId/999"))
+        .respond_with(ResponseTemplate::new(404).set_body_json(serde_json::json!({
+            "message": "File not found"
+        })))
+        .mount(&mock_server)
+        .await;
+
+    let result = client.get_file_info(1, 2, 999).await;
+    assert!(result.is_err());
+    if let Err(onspring::OnspringError::Api {
+        status_code,
+        message,
+    }) = result
+    {
+        assert_eq!(status_code, 404);
+        assert_eq!(message, "File not found");
+    } else {
+        panic!("Expected Api error");
+    }
+}