@foreach incorrectly iterates over string characters

﻿## Description

@foreach with IEnumerable cast matches string type, causing iteration over chars instead of treating as error.

## Steps to Reproduce

```razor
@foreach(var item in Context.Name) {
  Item: ${item}
}
```

If Context.Name is a string like 'Hello', it iterates H, e, l, l, o.

## Solution

Add explicit string check before IEnumerable cast in ProcessForeachBlock.