Use C# 14 extensions for polyfills in MetadataLoadContext.

[teo-tsirpanis]

Inspired by #126280.

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[Copilot]

Pull request overview

This PR updates System.Reflection.MetadataLoadContext to use C# extension members as polyfills, letting the codebase use modern Type APIs (e.g., IsSZArray, IsSignatureType) without helper extension-method calls.

Changes:

• Replaced IsSZArray(), IsVariableBoundArray(), IsGenericMethodParameter(), and IsSignatureType() extension-method calls with property-style access.
• Replaced MakeSignatureGenericType(...) helper usage with Type.MakeGenericSignatureType(...).
• Introduced NetStandard polyfills for these members using C# extension blocks and removed the NetCore emulator extension-method shim.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoModifiedType.cs	Switches SZ-array detection to property access.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoDefinitionType.cs	Uses IsSignatureType property and Type.MakeGenericSignatureType.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/General/TypeExtensions.netstandard.cs	Adds C# extension-member polyfills for NetStandard.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/General/TypeExtensions.net.cs	Removes NetCore NetCoreApiEmulators extension-method layer (file remains for leveled base types).
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/General/Assignability.cs	Converts SZ-array checks to property access.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/Runtime/SignatureTypeExtensions.cs	Converts signature-type and array-kind checks to property access.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/Runtime/BindingFlagSupport/MemberPolicies.cs	Converts SZ-array and generic-method-parameter checks to property access.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/DefaultBinder.cs	Converts signature-type checks to property access.

[jkotas]

Thanks

[jkotas]

/ba-g known issue #126354 that's not matched by build analysis for some reason