Add unit test for issue #7346 (custom attribute constructor crash on null array of enums)

[Copilot]

Plan to add unit test for issue #7346 (custom attribute constructor crash on null array of enums):

• Understand the issue: Issue Additional BotR Chapters #7346 was about a crash when a custom attribute constructor has a null array of enums
• Baseline setup: Build clr+libs successfully
• Configure PATH and verify SDK version
• Add a custom attribute class that accepts null enum array in constructor
• Add test method to verify the attribute can be retrieved without crashing
• Build the System.Runtime tests
• Run the tests to verify they pass - all CustomAttributeDataTests pass (8 tests)
• Apply code review suggestions - improved test coverage with multiple array scenarios
• Fix build break by adding using statement for ReadOnlyCollection
• Commit the changes

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add unit test for issue #7346 (custom attribute constructor crash on null array of enums)</issue_title>
<issue_description>Just need to wait for the fix in coreclr to propagate over to corefx.

https://github.com/dotnet/coreclr/issues/7346
</issue_description>

Comments on the Issue (you are @copilot in this section)

@karelz @atsushikan I unassigned it from you to reflect 'up for grabs'. If/when you work on it, please assign it to yourself. Thanks!

Fixes #18847

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[Copilot]

Pull Request Overview

This PR adds a unit test to verify that a crash issue with custom attribute constructors receiving null arrays of enums has been fixed. The issue (dotnet/coreclr#7346) caused crashes when custom attributes with null enum array parameters were processed through reflection.

• Adds a test case that applies a custom attribute with a null enum array parameter and verifies it can be retrieved safely
• Introduces supporting types (MyTestEnum and MyEnumArrayAttribute) for the test
• Validates that the fix prevents crashes and correctly handles null array values in custom attribute reflection