Fix SherpaOnnx finalizer crash with DllNotFoundException

[williamwwm]

Suppress fatal crashes caused by SherpaOnnx.OfflineTts finalizer calling SherpaOnnxDestroyOfflineTts when native DLL is unavailable.

Changes:

• PiperTextToSpeechClient: gracefully handle DllNotFoundException in constructor (TTS disabled gracefully), add GC.SuppressFinalize after dispose to prevent finalizer from calling native code during GC
• App.xaml.cs: improved OnDomainUnhandledException logging for SherpaOnnx native DLL errors
• Crash prevention relies on exe.config legacyUnhandledExceptionPolicy and GC.SuppressFinalize (IsTerminating is readonly in .NET 10)

Fixes repeated crashes: System.DllNotFoundException: Unable to load DLL 'sherpa-onnx-c-api' at SherpaOnnx.OfflineTts.Finalize()