Add a missing protocol conformance to ClientTransport (apple#4)

Add a missing protocol conformance to ClientTransport ### Motivation We lost URLSessionTransport's conformance to ClientTransport along the way somewhere. But that's kind of its main job :) ### Modifications Re-add the conformance. ### Result Now the transport can be used as a ClientTransport again. ### Test Plan Explicitly cast the test transport to be a ClientTransport, rather than using the concrete type. Reviewed by: simonjbeaumont Builds: ✔︎ pull request validation (5.8) - Build finished. ✔︎ pull request validation (5.9) - Build finished. ✔︎ pull request validation (nightly) - Build finished. ✔︎ pull request validation (soundness) - Build finished. apple#4

Author: czechboy0

Sources/OpenAPIURLSession/URLSessionTransport.swift

@@ -52,7 +52,7 @@ import Foundation
 /// The ``URLSessionTransport/Configuration-swift.struct`` type allows you to
 /// provide a custom URLSession and tweak behaviors such as the default
 /// timeouts, authentication challenges, and more.
-public struct URLSessionTransport {
+public struct URLSessionTransport: ClientTransport {
 
  /// A set of configuration values for the URLSession transport.
  public struct Configuration: Sendable {

Tests/OpenAPIURLSessionTests/URLSessionTransportTests.swift

@@ -59,7 +59,7 @@ class URLSessionTransportTests: XCTestCase {
  body: Data("👋".utf8)
  )
  )
- let transport = URLSessionTransport(
+ let transport: ClientTransport = URLSessionTransport(
  configuration: .init(session: MockURLProtocol.mockURLSession)
  )
  let request = OpenAPIRuntime.Request(