[6.x] Fix/15396 field error association

[jaygeorge]

Description of the Problem

When a publish form (or login) fails validation, the error is shown visually but never connected to the control that failed. Measured after a failed save on an empty required field:

• no aria-invalid="true"
• no aria-describedby on the input
• focus stays on BODY
• screen readers only hear a generic toast (“The given data was invalid”)

On a long blueprint that means walking the entire form to find the error. Field instructions have the same gap — they’re rendered as unlinked siblings. WCAG 2.1 SC 3.3.1 (and related 3.3.2).

Depends on fields having reliable ids (#15389).

What this PR Does

• Gives ErrorMessage / Description stable ids ({fieldId}-error, {fieldId}-description)
• Sets aria-describedby and aria-invalid on the field’s own control (id must match the Field id, so Table/List cells aren’t all marked invalid)
• Merges consumer aria-describedby with the field’s description/error ids
• Focuses the first invalid control after a failed submit (reveal.invalid(), including login)
• Includes the ui/Field label is not associated with its control on 37 of 48 call sites, including login #15389 Field label/id association work this depends on

Fixes #15396
Related: #15389

How to Reproduce

1. Add a required field to a collection blueprint
2. Create a new entry, leave the required field empty, click Save
3. Before this PR:

document.querySelectorAll('[aria-invalid="true"]').length   // 0
document.querySelectorAll('input[aria-describedby]').length // 0
document.activeElement.tagName                              // "BODY"

After this PR: the failing input is focused, has aria-invalid="true", and aria-describedby pointing at the error (and instructions when present).