Mark final to support Sendable

Tests/KingfisherTests/KingfisherOptionsInfoTests.swift

@@ -146,13 +146,13 @@ class KingfisherOptionsInfoTests: XCTestCase {
     }
 }
 
-class TestModifier: ImageDownloadRequestModifier {
+final class TestModifier: ImageDownloadRequestModifier {
     func modified(for request: URLRequest) -> URLRequest? {
         return nil
     }
 }
 
-class TestRedirectHandler: ImageDownloadRedirectHandler {
+final class TestRedirectHandler: ImageDownloadRedirectHandler {
     func handleHTTPRedirection(
         for task: Kingfisher.SessionDataTask, response: HTTPURLResponse, newRequest: URLRequest
     ) async -> URLRequest? {