Match CoreCLR behavior for NativeAOT string and array constructor invocation

[Copilot]

NativeAOT currently ignores the existing object when invoking string or array pseudo-constructors. CoreCLR instead surfaces NotSupportedException through TargetInvocationException.

• Implementation: Reject an existing target after argument validation, preserving exception wrapping.
• Coverage: Enable the existing string and array constructor tests on NativeAOT; retain their Mono exclusions.

Note

This description was generated with GitHub Copilot.

• Fixes [NativeAOT] Support invoking string and array constructors on existing objects #134900

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