Feature/vfp UI optimization

[Irwin1985]

• Added Environment.UserInteractive check to immediately fallback in non-UI environments (Services, IIS, etc).
• Refined LoadProvider to use physical file detection instead of AppDomain assembly checks to overcome .NET JIT Lazy Loading issues.
• Ensured MESSAGEBOX() and other UI functions triggered automatically if XSharp.VFP.UI.dll is present.

[Copilot]

Pull request overview

This PR optimizes the VFP UI provider loading mechanism in VfpUIService. The key improvement is an early-exit guard that prevents UI DLL loading in non-interactive environments (Windows Services, IIS), and cleans up the DLL detection logic by switching from AppDomain assembly checks to physical file existence checks.

Changes:

• Added Environment.UserInteractive check to immediately assign HeadlessUIProvider in non-interactive environments, avoiding unnecessary DLL discovery and loading
• Refactored LOCAL ... AS TYPE variable declarations to VAR inference within LoadProvider() for brevity

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