[cveld]

Part of #2463.

When several repositories (or worktrees) need a sign-in at the same time, the
prompts give no hint which request they belong to, so it is easy to dismiss or
accept one blindly.

This adds an ambient AuthPromptContext that the Azure Repos provider sets to
host/path before it asks Entra for a token. It is then shown in:

• the default-account prompt, in both the GUI view (For: <remote>) and the
  console prompt (Continue with current account (x) for <remote>?);
• the stub ProgressWindow that parents the Windows broker sign-in, which now
  also shows the working directory of the Git process that requested the
  credential (Requested from: <cwd>).

The context is an AsyncLocal, so concurrent requests in one process do not see
each other's value. If no context is set (any other provider), the prompts look
exactly as before; the progress window then only shows the working directory.

Scope

• Azure Repos / Entra only. GitHub and other providers' prompts are unchanged.
• It does not show the calling process (the "grandparent") that Can more context be provided in the pop up #2463 also asks
  for. The working directory is the closest available signal. Showing the process
  chain would need per-platform work and could be a follow-up.
• Because of this the PR says "Part of" and not "Closes".

Testing

• Added AuthPromptContextTests (flow into awaited work, isolation between
  concurrent flows, the progress window text with and without a request,
  DefaultAccountViewModel.HasContext).
• Ran the full solution test suite on Windows (dotnet test git-credential-manager.slnx,
  with DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=1 because the six
  Trace2MessageTests.BuildTimeSpan cases fail under a decimal-comma culture on my machine):
  all projects pass.
• Rendered the real ProgressWindow and DefaultAccountView with sample data
  (short and very long remotes and paths) to check that the text wraps and the
  windows grow as expected.
• Not yet verified: macOS and Linux (the GUI view and console prompt changes are
  platform-neutral, but I only ran Windows), and the progress window during a real
  broker sign-in through git-credential-manager.exe.

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