docs(readme): add close method override in UserAgentClient example

[EchoEllet]

It seems that close() is a no-op without an override in this case:

import 'package:http/http.dart' as http;
import 'package:http/io.dart';

// Copied from https://pub.dev/packages/http#using
class UserAgentClient(final String userAgent, final http.Client _inner) extends http.BaseClient {

  @override
  Future<http.StreamedResponse> send(http.BaseRequest request) {
    request.headers['user-agent'] = userAgent;
    return _inner.send(request);
  }
}

void main() {
  final http.BaseClient client = UserAgentClient('user-agent', IOClient());
  client.close(); // No-op. Does not call IOClient.close()
}

I'm not sure if I misunderstood something or am consuming the API in the wrong way.

If the intended way to close the HTTP client is:

void main() {
  final http.BaseClient innerClient = IOClient();
  
  final http.BaseClient client = UserAgentClient('user-agent', innerClient);
   // Send requests using "client"...

  innerClient.close();
}

Then the current example code is as intended. But even then, I would throw a Dart error when calling UserAgentClient.close(), to avoid silent programming bugs.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

Note: The Dart team is trialing Gemini Code Assist. Don't take its comments as final Dart team feedback. Use the suggestions if they're helpful; otherwise, wait for a human reviewer.

[EchoEllet]

If the intended way to close the HTTP client is:

It is worth noting that RetryClient (from the http package) calls _inner.close() (source). So I don't think the current example is as intended.