Add code fix for MSTEST0040 — `AvoidUsingAssertsInAsyncVoidContext`

[Evangelink]

Priority: High
Estimated Effort: Small–Medium

AvoidUsingAssertsInAsyncVoidContextAnalyzer (MSTEST0040) warns when Assert calls appear in async void methods. A code fix can offer "Change return type to Task" to convert async void to async Task, which is the canonical remediation.

File to create: `src/Analyzers/MSTest.Analyzers.CodeFixes/AvoidUsingAssertsInAsyncVoidContextFixer.cs

[Evangelink]

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Note

The pull request was not created — a fallback review issue was created instead due to protected file changes: #8059

🤖 This is an automated response from Test Improver.

When implementing AvoidUsingAssertsInAsyncVoidContextFixer, the tests in AvoidUsingAssertsInAsyncVoidContextAnalyzerTests.cs currently use EmptyCodeFixProvider. Once the fixer lands, updating the VerifyCS alias to reference the real fixer class and adding fixedCode assertions is the standard pattern in this repo:

// Before (current)
using VerifyCS = MSTest.Analyzers.Test.CSharpCodeFixVerifier<
    MSTest.Analyzers.AvoidUsingAssertsInAsyncVoidContextAnalyzer,
    Microsoft.CodeAnalysis.Testing.EmptyCodeFixProvider>;

// After
using VerifyCS = MSTest.Analyzers.Test.CSharpCodeFixVerifier<
    MSTest.Analyzers.AvoidUsingAssertsInAsyncVoidContextAnalyzer,
    MSTest.Analyzers.AvoidUsingAssertsInAsyncVoidContextFixer>;

Tests then call VerifyCS.VerifyCodeFixAsync(code, fixedCode) where fixedCode is the expected output after applying the "Change return type to Task" fix. See AvoidOutParameterOnAssertIsInstanceOfTypeFixerTests.cs for a reference example of this pattern.

I'm tracking this to add comprehensive code fix tests once the fixer is implemented.

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