[@mantine/core] Input: Add keepErrorMounted prop for persistent error element (aria-live)

[phamvutinh]

Closes #8932

Problem

Input.Wrapper (and therefore all inputs built on it — Select, TextInput, Textarea, etc.) unmounts the error element entirely when the error prop is falsy. This makes it impossible to use Mantine's built-in error node as a reliable aria-live region for field-level validation: each time an error appears a brand-new node is mounted instead of an existing one having its text updated, so screen reader announcements become inconsistent. Consumers currently have to render a second custom error element just to make validation accessible.

Solution

Add an opt-in keepErrorMounted prop (default false, fully backwards compatible). When set, the error element stays mounted even when there is no error, rendering empty content. Its text is then updated in place when an error appears/disappears, giving consumers a stable live-region host:

<TextInput
  label="Email"
  keepErrorMounted
  errorProps={{ 'aria-live': 'polite' }}
  error={hasError ? 'Required field' : null}
/>

Changes

• Input.Wrapper: new keepErrorMounted prop on __InputWrapperProps; error node renders when hasError || keepErrorMounted, with empty content when there is no error. aria-describedby references the error id whenever the node is mounted.
• use-input-props: route keepErrorMounted to the wrapper so all inputs (Select, TextInput, …) inherit it.
• get-input-offsets: reserve the error slot spacing when keepErrorMounted so layout doesn't shift as the error toggles.
• Docs: new "keepErrorMounted prop" section in input.mdx.
• Tests added for InputWrapper and get-input-offsets.

Open question for maintainer

I went with an opt-in prop (no behavior change, no layout regression). aria-live is left to the consumer via errorProps rather than applied automatically — happy to adjust the approach (e.g. a documented pattern only, or auto aria-live) based on your preference.

Checks

npm run typecheck, npx oxlint, npm run build, and npm run jest for Input/Select all pass.

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