Add regression test for ParseAttributeArgumentList with invalid input (fixes #8699)

[Copilot]

Summary

This PR adds a regression test to verify that SyntaxFactory.ParseAttributeArgumentList properly handles invalid input without throwing a NullReferenceException.

Issue

Issue #8699 reported that calling SyntaxFactory.ParseAttributeArgumentList("somethingWithoutBrackets") would throw a NullReferenceException when given text without parentheses.

Resolution

Testing confirms that this issue is already fixed in the current codebase. The method now properly handles invalid input by:

1. Returning a non-null AttributeArgumentListSyntax object
2. Creating missing tokens for OpenParenToken and CloseParenToken
3. Including appropriate diagnostics for the unexpected tokens

Changes

Added TestParseAttributeArgumentListWithInvalidString test that verifies:

• The method returns a valid syntax node (not null)
• Diagnostics are generated for the invalid input
• The structure contains missing parenthesis tokens as expected
• No exception is thrown

Example

// This no longer throws NullReferenceException
var result = SyntaxFactory.ParseAttributeArgumentList("somethingWithoutBrackets");

// Returns a valid node with diagnostics
Assert.NotNull(result);
Assert.True(result.GetDiagnostics().Any());
Assert.True(result.OpenParenToken.IsMissing);
Assert.True(result.CloseParenToken.IsMissing);

This test serves as documentation that the issue has been resolved and prevents future regression.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>c# SyntaxFactory.ParseAttributeArgumentList throws NullReferenceException with invalid string</issue_title>
<issue_description>Version Used:
typeof(SyntaxFactory).Assembly.ImageRuntimeVersion = v4.0.30319

Steps to Reproduce:
in c# interactive:

1. #r "Microsoft.CodeAnalysis.CSharp.dll"
2. using Microsoft.CodeAnalysis.CSharp;
3. SyntaxFactory.ParseAttributeArgumentList("somethingWithoutBrackets")
4. Then this exception is being thrown:
   Object reference not set to an instance of an object.

• Microsoft.CodeAnalysis.CSharp.Syntax.InternalSyntax.SyntaxParser.AddError(TNode, Microsoft.CodeAnalysis.CSharp.ErrorCode, object[])
• Microsoft.CodeAnalysis.CSharp.Syntax.InternalSyntax.LanguageParser.ConsumeUnexpectedTokens(TNode)
• Microsoft.CodeAnalysis.CSharp.SyntaxFactory.ParseAttributeArgumentList(string, int, Microsoft.CodeAnalysis.ParseOptions, bool)

Expected Behavior:
some kind of ParseException, e.g. UnexpectedTokenException
Actual Behavior:
NullReferenceException with stacktrace.
</issue_description>

<agent_instructions>Copilot, can you first add a test to see if htis issue still reproes. If so, fix. If not, we can just submit the test to demonstrate that this issue can be closed.</agent_instructions>

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

Fixes #23877

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