Add unique exit code for mount authentication failures

[tyrielv]

Summary

When gvfs mount fails due to authentication, both GVFS.Mount.exe and gvfs.exe now exit with ReturnCode.AuthenticationError (9) instead of GenericError (3), enabling callers to distinguish auth failures from other errors.

Changes

• ReturnCode.AuthenticationError = 9 — new enum value
• GitAuthentication.TryInitializeAndQueryGVFSConfig — added out bool isAuthFailure parameter that explicitly classifies auth failures:
  • Credential fetch failure → auth failure
  • 401/403 on credentialed retry → auth failure
  • Other failures (500, timeout, network) → not auth failure
• InProcessMount — uses isAuthFailure to select ReturnCode.AuthenticationError when calling FailMountAndExit
• GVFSPlatform.StartBackgroundVFS4GProcess — returns Process (was void) so MountVerb can read the mount process exit code
• MountVerb — captures the mount process handle, waits up to 1s for exit on failure, and propagates the exit code to the caller

Testing

• All 811 unit tests pass
• Auth failure classification tested against four scenarios:
  1. Non-401 on anonymous probe → NOT auth (correct)
  2. 401 + credential fetch fails → IS auth (correct)
  3. 401 + creds obtained + retry returns 401/403 → IS auth (correct)
  4. 401 + creds obtained + retry returns 500/timeout → NOT auth (correct)
• Verified that Azure DevOps returns proper 401 (not 302/203 redirect) for the anonymous config probe because HttpRequestor sends the X-TFS-FedAuthRedirect: Suppress header on all requests

Future Work

Concurrent mount exit code propagation: When two gvfs mount commands run simultaneously, the second mount process (GVFS.Mount.exe) fails to acquire the mount lock and exits with MountAlreadyRunning (8). If the first mount also fails (e.g., due to auth), MountVerb for the second invocation currently reports MountAlreadyRunning rather than the first mount's actual failure reason. A follow-up PR will address this by having the mount process persist its exit status so that concurrent mount attempts can propagate the correct error.

Early exit detection: Currently, if GVFS.Mount.exe exits before starting its named pipe server (as happens with auth failures), MountVerb still waits for the full named pipe connection timeout (60s, or 300s unattended) before checking the process exit code. A follow-up PR will add early exit detection so MountVerb can fail fast when the mount process has already terminated.