Revert "allow null CancellationToken"

The null errors were from httpClient, not the cancellation token. They
were happening because HgServiceTests mocks IHttpClientFactory,
returning null when code tries to access HttpClient. We'll need to mock
IHttpClientFactory in a slightly more detailed way.

backend/LexBoxApi/Services/HgService.cs

@@ -286,13 +286,11 @@ public async Task InvalidateDirCache(string code)
         return int.TryParse(str, out int result) ? result : null;
     }
 
-    private async Task<HttpContent> ExecuteHgCommandServerCommand(string code, string command, CancellationToken? token)
+    private async Task<HttpContent> ExecuteHgCommandServerCommand(string code, string command, CancellationToken token)
     {
         var httpClient = _hgClient.Value;
         var baseUri = _options.Value.HgCommandServer;
-        var response = token == null
-            ? await httpClient.GetAsync($"{baseUri}{code}/{command}", HttpCompletionOption.ResponseHeadersRead)
-            : await httpClient.GetAsync($"{baseUri}{code}/{command}", HttpCompletionOption.ResponseHeadersRead, token.Value);
+        var response = await httpClient.GetAsync($"{baseUri}{code}/{command}", HttpCompletionOption.ResponseHeadersRead, token);
         response.EnsureSuccessStatusCode();
         return response.Content;
     }