"remote authentication required but no callback set" in pull after upgrading from 0.28 to 0.29

@bording we get the error "remote authentication required but no callback set" after upgrading from 0.28 to 0.29 when connecting to a company internal azure devops server.

This works great with the old version using windows authentication.

Reproduction steps

• connect to a company internal azure devops server via windows authentication

      Signature signature = GetSignature();
      PullOptions pullOptions = new()
      {
        MergeOptions = new() { FileConflictStrategy = CheckoutFileConflictStrategy.Theirs }
      };
      MergeResult mergeResult = Commands.Pull(Repository, signature, pullOptions);

Expected behavior

Success

Actual behavior

LibGit2Sharp.LibGit2SharpException
HResult=0x80131500
Message=remote authentication required but no callback set
Source=LibGit2Sharp
StackTrace:
at LibGit2Sharp.Core.Ensure.HandleError(Int32 result) in /_/LibGit2Sharp/Core/Ensure.cs:line 154

Version of LibGit2Sharp (release number or SHA1)

0.29

Operating system(s) tested; .NET runtime tested

Windows 11, .NET 4.8

Seems to be related to #2065 and #2087

先從 Core/Ensure.cs 第154行的 Pull 呼叫和錯誤路徑入手，然後比較 0.28 和 0.29 版本之間的驗證行為。檢視相關 issue #2065 和 #2087，並在使用 .NET 4.8 的 Windows 內部 Azure DevOps 伺服器上重現該問題。當顯示的 pull 使用 Windows 驗證成功執行且沒有 missing-callback exception 時，即表示完成。