[browser] [wasm] Remove manual ThrowIfCancellationRequested from ReadAsync response streaming

[campersau]

While working on #91699 I noticed that there was one manual call to ThrowIfCancellationRequested in ReadAsync when doing response streaming, which means that this cancellation token does not go through the CancelationHelper which means that this won't abort the actual fetch request.

It normally shouldn't be an issue because developers are supposed to dispose the HttpResponseMessage / Stream which both would abort the fetch request.

With this PR ReadAsync would also abort the fetch request:

var request = new HttpRequestMessage(HttpMethod.Get, "https://someurl");
request.SetBrowserResponseStreamingEnabled(true);

var response = await new HttpClient().SendAsync(request, HttpCompletionOption.ResponseHeadersRead); // no using / dispose
var stream = await response.Content.ReadAsStreamAsync(); // no using / dispose
await stream.ReadAsync(Array.Emtpy<byte>(), new CancellationToken(true)); // now aborts fetch!

---

All other methods which could have called ThrowIfCancellationRequested also didn't and correctly go through CancelationHelper.

runtime/src/libraries/System.Net.Http/src/System/Net/Http/BrowserHttpHandler/BrowserHttpHandler.cs

Line 455 in d859279

private async ValueTask<byte[]> GetResponseData(CancellationToken cancellationToken)

runtime/src/libraries/System.Net.Http/src/System/Net/Http/BrowserHttpHandler/BrowserHttpHandler.cs

Line 504 in d859279

protected override Task SerializeToStreamAsync(Stream stream, TransportContext? context, CancellationToken cancellationToken)

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Issue Details

---

While working on #91699 I noticed that there was one manual call to ThrowIfCancellationRequested in ReadAsync when doing response streaming, which means that this cancellation token does not go through the CancelationHelper which means that this won't abort the actual fetch request.

It normally shouldn't be an issue because developers are supposed to dispose the HttpResponseMessage / Stream which both would abort the fetch request.

With this PR this would also abort the fetch request:

var request = new HttpRequestMessage(HttpMethod.Get, "https://someurl");
request.SetBrowserResponseStreamingEnabled(true);

var response = await new HttpClient().SendAsync(request, HttpCompletionOption.ResponseHeadersRead); // no using / dispose
var stream = await response.Content.ReadAsStreamAsync(); // no using / dispose
await stream.ReadAsync(Array.Emtpy<byte>(), new CancellationToken(true)); // now aborts fetch!

Author:	campersau
Assignees:	-
Labels:	area-System.Net.Http, community-contribution
Milestone:	-