Fix false positive in MissedSelectOpportunity when foreach body uses await

[Copilot]

The cs/linq/missed-select query incorrectly flagged foreach loops whose first statement used await — loops that cannot be refactored to .Select(...) since LINQ's Select operates on IEnumerable<T> and does not support async lambdas.

// Previously flagged — cannot use Select because of await
foreach (var locator in contactSelectors.Select(s => Page.Locator(s)))
{
    var elements = await locator.CountAsync(); // ← await here blocks Select refactor
    if (elements > 0) { ... }
}

Changes

• csharp/ql/lib/Linq/Helpers.qll: Added exclusion to missedSelectOpportunity — the predicate no longer holds when the first statement's initializer contains an AwaitExpr. Updated doc comment accordingly.

• csharp/ql/test/query-tests/Linq/MissedSelectOpportunity/: New test covering:

  • BAD: plain variable mapping (should be flagged)
  • GOOD: variable mapping whose initializer uses await (should not be flagged)

[hvitved]

@copilot : Please address this.

[hvitved]

Should use inline test expectations, i.e.

postprocess:
 - utils/test/InlineExpectationsTestQuery.ql

This also means adding // $ Alert marker comment in the test file.