[@mantine/core] Textarea: Fix accessibility false positives for autosize (#8712)

[dsuzener]

The hidden mirror <textarea> used internally by react-textarea-autosize (when autosize is enabled) triggers the error “Form control does not have a corresponding label”

Although the mirror element has aria-hidden="true" and tabindex="-1" and is not interactive or visible to users, tools like WAVE still flag it because it lacks an accessible name.

Solution

This PR adds a runtime patch that detects the hidden mirror <textarea> based on aria-hidden="true, tabindex="-1", and some styles. Then, it adds role="presentation" and aria-label="autosize measurement mirror textarea".

The patch runs only when autosize is enabled and does not affect other textareas.

Notes

• This is a workaround until react-textarea-autosize includes proper accessibility attributes on the mirror element.
• Screen readers and real user accessibility are not impacted.
• The change eliminates WAVE false positives on the Textarea documentation page.