fix(QInput,QField): expose error state to assistive tech

[web3blind]

Fixes #17306.

This adds the missing ARIA error-state relationship for Quasar fields so assistive technologies can identify invalid controls and reach the rendered error message.

What changed

• Adds a stable id to the rendered field error message container while an error is active.
• Applies aria-invalid="true", aria-errormessage, and aria-describedby to native QInput controls in error state.
• Preserves existing aria-describedby values instead of replacing them.
• Exposes ariaInvalid, ariaDescribedby, and ariaErrormessage in the QField control slot scope so custom controls can wire the same relationship.
• Documents the new control slot scope values in the QField API metadata.
• Adds focused tests for native QInput controls and custom QField control slots.

Validation

• pnpm --filter quasar build
• pnpm --filter quasar test -- --run ui/src/components/input/QInput.test.js ui/src/components/field/QField.test.js
  • Test Files 101 passed (101)
  • Tests 1063 passed (1063)
• pnpm exec oxlint --no-error-on-unmatched-pattern ui/src/components/input/QInput.js ui/src/composables/private.use-field/use-field.js ui/src/components/input/QInput.test.js ui/src/components/field/QField.test.js
  • Found 0 warnings and 0 errors.
• git diff --check

Note: full pnpm lint:ui:check was blocked in my local checkout by a generated-file/environment issue unrelated to these changes: ui/playground/.quasar/tsconfig.json was missing.