Fix BL0012 false positive on await foreach and await using - #68720

Problem

BL0012 reports StateHasChanged calls that are required, whenever the enclosing method's only awaits are await foreach or await using.

protected override async Task OnInitializedAsync()
{
    await foreach (var item in LoadAsync())
    {
        _items.Add(item);
        StateHasChanged();   // BL0012: "unnecessary in method 'OnInitializedAsync'"
    }
}

The call is what paints each item as it arrives. Accepting the offered code fix removes it, and the list then stays empty until the stream completes and appears in one batch.

Cause

StateHasChangedAnalyzer located the first and last await by collecting AwaitExpressionSyntax nodes:

var awaitExpressions = body.DescendantNodes(...).OfType<AwaitExpressionSyntax>()

Three forms do not produce an AwaitExpressionSyntax. They carry their await as a keyword token on the statement instead:

A method whose only awaits take one of those forms therefore looked await-free. The analyzer took its Count == 0 branch, which treats every call in the method as redundant on the grounds that ComponentBase renders once the method returns.

Fix

Track the region the method can suspend in, rather than a set of await positions. Each construct contributes the span over which it can yield:

The reporting rule is unchanged: a call is redundant when it sits outside that region, so behaviour for ordinary awaits stays as it was.

The last row is why a call after an await using declaration is no longer reported. Its disposal runs at the end of the enclosing block, so the call still has an await after it and ComponentBase has not rendered yet.

Tests

Five added to StateHasChangedAnalyzerTest:

• three asserting no diagnostic for a call inside await foreach, inside an await using block, and after an await using declaration
• two asserting a call before or after an await foreach is still reported

With the analyzer change reverted the three fail and the two pass.

Found while validating #68484.