Use System.Runtime.InteropServices.Marshalling.ComVariant instead of custom VARIANT types in libraries

[jkoritzinsky]

OleDb and DirectoryServices were using their own representations of the Win32 VARIANT and PROPVARIANT types. Unify on the ComVariant type as it follows interop best practices to map the Win32 VARIANT type.

[Copilot]

Pull request overview

This pull request replaces custom VARIANT and PROPVARIANT type definitions in System.DirectoryServices and System.Data.OleDb libraries with the standard ComVariant type from System.Runtime.InteropServices.Marshalling. This unification follows interop best practices for representing Win32 VARIANT types.

Key changes:

• Removed custom Variant and PROPVARIANT struct definitions and replaced them with ComVariant
• Consolidated VARIANT and PROPVARIANT handling in OleDb to use the same cleanup code path
• Updated all usages to call ComVariant.Create() and ComVariant.Dispose() instead of manual struct initialization and native cleanup functions

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/libraries/System.DirectoryServices/src/System/DirectoryServices/DirectoryEntry.cs	Replaced manual VARIANT initialization with ComVariant.Create(true)
src/libraries/System.DirectoryServices/src/System.DirectoryServices.csproj	Removed references to VariantClear and VariantInit interop files
src/libraries/System.DirectoryServices/src/Interop/UnsafeNativeMethods.cs	Removed custom Variant struct and updated IAdsObjectOptions2 to use ComVariant
src/libraries/System.DirectoryServices/src/Interop/EnumVariant.cs	Updated to use ComVariant with proper disposal pattern
src/libraries/System.Data.OleDb/src/System.Data.OleDb.csproj	Removed references to VariantClear and PropVariantClear interop files
src/libraries/System.Data.OleDb/src/SafeHandles.cs	Removed entire PROPVARIANT struct definition and related types (307 lines)
src/libraries/System.Data.OleDb/src/RowBinding.cs	Consolidated VARIANT and PROPVARIANT cleanup to use single code path with ComVariant
src/libraries/System.Data.OleDb/src/PropertyInfoSet.cs	Updated to use ComVariant.Dispose() instead of VariantClear
src/libraries/System.Data.OleDb/src/OleDb_Util.cs	Updated SizeOf_Variant calculation to use Unsafe.SizeOf()
src/libraries/System.Data.OleDb/src/OleDb_Enum.cs	Updated D_PropVariant size to use sizeof(ComVariant)
src/libraries/System.Data.OleDb/src/DbPropSet.cs	Updated to use ComVariant.Dispose() instead of VariantClear
src/libraries/System.Data.OleDb/src/DbBindings.cs	Updated comments to reference ComVariant instead of PROPVARIANT
src/libraries/Common/src/Interop/Windows/OleAut32/Interop.VariantInit.cs	Deleted file - no longer needed with ComVariant

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