## Description
When RadEditor switches to HTML mode, a `<textarea>` with class `reTextArea` is dynamically created. This textarea lacks proper accessibility attributes, causing WCAG accessibility tools (e.g., Wave) to flag it as an unlabeled form control.

## Steps to Reproduce
1. Create a RadEditor with `EnableAriaSupport="true"`
2. Switch to HTML mode
3. Run Wave or similar accessibility checker
4. Observe the textarea is flagged as "missing form label"

## Expected Behavior
When `EnableAriaSupport` is enabled, the HTML mode textarea should have:
- `aria-label` attribute to identify its purpose
- `aria-hidden="true"` when in Design mode (hidden state)

## Fix
Added ARIA attributes in `_getHtmlModeTextarea()` and toggle `aria-hidden` in `_showTextArea()` based on `EnableAriaSupport` setting.