Ignore AssociatedSourceType on UnmanagedCallersOnly in CoreCLR

[Copilot]

CoreCLR rejected [UnmanagedCallersOnly] attributes containing the newer AssociatedSourceType named argument, throwing META_E_CA_UNKNOWN_ARGUMENT when setting up unmanaged callbacks. Assemblies valid for NativeAOT could therefore fail under CoreCLR.

• Runtime

  • Accept AssociatedSourceType in CoreCLR’s hand-written UnmanagedCallersOnlyAttribute parser.
  • Parse and ignore the value, matching the property’s NativeAOT-only semantics.

• Regression coverage

  • Add a CoreCLR interop test for an unmanaged function pointer call to a method annotated with:

    [UnmanagedCallersOnly(AssociatedSourceType = typeof(UnmanagedCallersOnlyBasicTest))]

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[MichalStrehovsky]

@copilot same problem with mono: * Assertion at /__w/1/s/src/mono/mono/metadata/marshal.c:3478, condition `is_ok (error)' not met, function:mono_marshal_set_callconv_from_unmanaged_callers_only_attribute, Binary format of the specified custom attribute was invalid. assembly: type:UnmanagedCallersOnlyBasicTest member:(null)

[Copilot]

@copilot same problem with mono: * Assertion at /__w/1/s/src/mono/mono/metadata/marshal.c:3478, condition `is_ok (error)' not met, function:mono_marshal_set_callconv_from_unmanaged_callers_onl...

Addressed for Mono in baec198. The Mono path now parses the UnmanagedCallersOnly custom attribute blob directly for CallConvs and skips other named arguments such as AssociatedSourceType; the Mono-flavored UnmanagedCallersOnlyBasicTest passes locally.

[MichalStrehovsky]

@copilot this is a lot of new code in Mono. we only need this to support this narrow path with the existing limitations (e.g. multiple callconvs do not work). can you address the specific issue with the extra property only? I don't want 250 lines of new code for something that was handled by 12 lines. keep the diff against main branch minimal.