feat(field): use aria-errormessage for error text association

[justintoman]

Field.ErrorText is currently associated with the control via aria-describedby. I believe aria-errormessage is the semantically correct attribute for this relationship. It exists specifically to identify error messages and pairs with aria-invalid.

Testing Library and Playwright both ship toHaveAccessibleErrorMessage() as a first-class assertion that reads aria-errormessage. I take that to mean this API should be stable and safe to support. I discovered this because I was attempting to write unit tests in my project with both of those and realized I couldn't use toHaveAccessibleErrorMessage() out of the box with Ark UI.

This change should be minimal because the infrastructure already correctly sets aria-invalid and determines hasErrorText with a MutationObserver. The error text ID just needs to move from aria-describedby to aria-errormessage, leaving aria-describedby for helper text exclusively, which I believe is the correct semantic split.

Thank you!

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