Fix MA0042 comment placement for await using

[meziantou]

Why

MA0042's code fix for using var did not preserve leading trivia correctly. When a comment preceded the statement, the fix inserted await on its own line before the comment instead of producing await using.

What changed

• Added a regression test, Using_Diagnostic1_WithComment, in DoNotUseBlockingCallInAsyncContextAnalyzer_AsyncContextTests.
• Updated ReplaceWithAwaitUsing in DoNotUseBlockingCallInAsyncContextFixer to preserve leading trivia by transferring the using token's leading trivia to the new await token and clearing it from using.

Result

The code fix now produces:

// MA0042 "Prefer using 'await using'"
await using var connection = new SqlConnection("");

instead of splitting await onto a separate line.