Fix "Generate abstract method" adding disallowed async modifier

[Copilot]

• Understand the issue: Generate abstract method incorrectly adds async modifier
• Fix the code in AbstractGenerateParameterizedMemberService.SignatureInfo.cs to prevent async modifier when isAbstract is true
• Add test case to verify abstract methods returning Task don't get async modifier
• Build and test the changes
• Verify the fix works correctly
• Add WorkItem attribute to test for issue tracking

Summary

Fixed the issue where "Generate abstract method" was incorrectly adding the async modifier to abstract methods that return Task-like types. Abstract methods cannot have the async modifier because they don't have a body (CS1994 error).

Changes Made:

1. AbstractGenerateParameterizedMemberService.SignatureInfo.cs: Modified line 108 to only apply the async modifier when the method is NOT abstract AND returns a Task-like type
2. GenerateMethodTests.cs: Added test TestGenerateAbstractMethodReturningTask with WorkItem attribute to verify abstract methods returning Task don't get the async modifier

The fix ensures that WithAsync() is only set to true when !isAbstract && knownTypes.IsTaskLike(returnType), preventing the CS1994 error.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>"Generate abstract method" adds disallowed async modifier</issue_title>
<issue_description>Version Used: 18.0.0 Insiders [11104.47]

Steps to Reproduce:

Open the lightbulb menu on the identifier M2 and select "Generate abstract method 'M2'".

abstract class C
{
    async Task M()
    {
        await M2();
    }
}

Expected Behavior:
A valid abstract method declaration is inserted:

protected abstract Task M2();

Actual Behavior:
The async modifier is added to the method declaration, resulting in error CS1994

protected abstract async Task M2(); // CS1994: The 'async' modifier can only be used in methods that have a body.
```</issue_description>

<agent_instructions>Fix is likely in src\Analyzers\Core\CodeFixes\GenerateParameterizedMember\AbstractGenerateParameterizedMemberService.SignatureInfo.cs

Please add tests in src\Analyzers\CSharp\Tests\GenerateMethod\GenerateMethodTests.cs</agent_instructions>

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

<comments>
</comments>

Fixes #80628

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[CyrusNajmabadi]

@copilot Please add a WorkItem("#80628") so we can track which issue this is a test for.

[Copilot]

Added in commit cfe1b13.

[CyrusNajmabadi]

@davidwengier ptal.