ProjFS: tolerate ACCESS_DENIED on FilterAttach, remove dead bundled-driver code

[tyrielv]

Summary

Updates to improve resilience around GVFS failed to attach filter driver with result 2147942405 (0x80070005 / ACCESS_DENIED).

The ProjFS filter driver and native library are no longer bundled with the GVFS installer — ProjFS is required as a Windows Optional Feature (available since Windows 10 1809, our minimum supported OS). This PR removes the dead code paths that referenced those bundled files and fixes the FilterAttach ACCESS_DENIED error.

Changes

Fix FilterAttach ACCESS_DENIED

• Add TryAttachToVolume() that tolerates ACCESS_DENIED when the ProjFS service is already running (the filter is already attached to the volume but the caller lacks SE_LOAD_DRIVER_PRIVILEGE).
• The service-side EnableAndAttachProjFSHandler.Run() now uses TryAttachToVolume instead of raw TryAttach, matching the tolerance already present in the client-side IsReady() path.
• Fix Run() to preserve the first error from TryEnablePrjFlt instead of potentially overwriting it with a subsequent attach error.

Remove dead non-inbox ProjFS code paths

• Remove TryInstallProjFSViaINF (referenced {app}\Filter\prjflt.inf which is no longer shipped).
• Remove TryCopyNativeLibIfDriverVersionsMatch and supporting methods (referenced {app}\Filter\prjflt.sys and {app}\ProjFS\ProjectedFSLib.dll which are no longer shipped).
• Simplify TryEnableOrInstallDriver to always use the Windows Optional Feature path; keep build number logging as best-effort telemetry.
• Simplify IsNativeLibInstalled to require System32; warn if stale app-local DLL found from a legacy non-inbox install.

Improve error messaging

• When ProjFS native library is missing, error now includes the PowerShell command to enable the optional feature.
• When ProjFS cannot be enabled, error directs user to the optional feature instead of referencing non-existent bundled files.

Testing

• Unit tests updated and passing (3/3 ProjFSFilterTests).
• Installer Setup.iss legacy cleanup left intact (still needed for old→new upgrades).

Stats

3 files changed, 80 insertions, 412 deletions