Remove dead NativeAOT dynamic-delegate path from System.Delegate

[Copilot]

main PR N/A

Description

Delegate.IsDynamicDelegate() in NativeAOT checked an impossible condition (GetThunk(MulticastThunk) == IntPtr.Zero), leaving an unreachable DynamicInvokeImpl branch. This change removes that dead representation check and keeps DynamicInvoke on the normal reflection-based invoke path.

• Core runtime cleanup

  • Deleted IsDynamicDelegate().
  • Removed unreachable dynamic-delegate branch from DynamicInvokeImpl.
  • Removed obsolete CombineImpl guard that depended on IsDynamicDelegate().

• Behavior-preserving simplification

  • Retained ReflectionAugments.GetDelegateDynamicInvokeInfo(GetType()).
  • Retained invoke-time exception wrapping (wrapInTargetInvocationException: true).
  • Retained debugger-step annotation behavior.

• Scope control

  • Reviewed NativeAOT/runtime/compiler/libraries/tests for code solely tied to this obsolete path.
  • Removed only code directly coupled to that path; no unrelated thunk machinery changes.

[DebuggerGuidedStepThroughAttribute]
protected virtual object? DynamicInvokeImpl(object?[]? args)
{
    DynamicInvokeInfo dynamicInvokeInfo = ReflectionAugments.GetDelegateDynamicInvokeInfo(GetType());

    object? result = dynamicInvokeInfo.Invoke(_target, _methodPtr,
        args, binderBundle: null, wrapInTargetInvocationException: true);
    DebugAnnotations.PreviousCallContainsDebuggerStepInCode();
    return result;
}

Customer Impact

Lower maintenance risk in delegate invocation code by removing unreachable logic and redundant branching in a core runtime type.

Regression

Not a recent regression fix; this is dead-code removal and path simplification.

Testing

Focused validation was run to ensure normal DynamicInvoke behavior remains intact (including targeted delegate DynamicInvoke coverage) and CoreLib/NativeAOT build viability was rechecked.

Risk

Low. Narrowly scoped to deleting unreachable logic and preserving the existing active invocation path.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

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[MichalStrehovsky]

@MichalPetryka the merge conflict you requested is now in