Validation errors are not programmatically associated with the field that failed

[bpmore]

Bug description

When a publish form fails validation, the error is shown visually but never connected to the control that failed.

Measured immediately after a failed save on an entry with an empty required field:

elements with aria-invalid="true"   ->  0
inputs with aria-describedby       ->  0
document.activeElement             ->  BODY
visible error text                 ->  "This field is required."
toast                              ->  role="status" aria-live="polite", "The given data was invalid."

So a screen reader user hears a generic "The given data was invalid", focus stays where it was, and nothing identifies which field is at fault. On a long blueprint that means walking the entire form to find the error.

ErrorMessage.vue renders no id, so nothing could reference it even if the wiring existed:

https://github.com/statamic/cms/blob/v6.31.0/resources/js/components/ui/ErrorMessage.vue
https://github.com/statamic/cms/blob/v6.31.0/resources/js/components/ui/Field.vue#L104 (error rendered as an unlinked sibling)

The login form has the same shape: "These credentials do not match our records." is rendered outside any live region, and the email input gets no aria-invalid.

WCAG 2.1 SC 3.3.1 Error Identification (Level A). Related: SC 3.3.2 Labels or Instructions, since field instructions are also rendered as an unlinked sibling via Description.

How to reproduce

1. composer create-project statamic/statamic
2. Add a required field to a collection blueprint
3. Create a new entry, leave the required field empty, click Save
4. In the console:

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

Suggested fix

1. Give ErrorMessage an id derived from the field id (${fieldId}-error).
2. Set aria-describedby and aria-invalid="true" on the control when its field has errors.
3. Move focus to the first invalid control after a failed submit.

Field instructions should join the same aria-describedby list, which closes 3.3.2 in the same change.

This depends on fields reliably having ids, which is the subject of #15389.

Environment

Environment
Laravel Version: 13.30.1
PHP Version: 8.4.23
Composer Version: 2.10.2
Environment: local
Debug Mode: ENABLED
Maintenance Mode: OFF
Timezone: UTC
Locale: en

Cache
Config: NOT CACHED
Events: NOT CACHED
Routes: NOT CACHED
Views: CACHED

Drivers
Broadcasting: log
Cache: file
Database: sqlite
Logs: stack / single
Mail: log
Queue: sync
Session: file

Storage
public/storage: NOT LINKED

Statamic
Addons: 0
License Key: Not set
Sites: 1
Stache Watcher: Enabled (auto)
Static Caching: Disabled
Version: 6.31.0 PRO

Installation

Fresh statamic/statamic site via CLI

Additional details

Found during a WCAG 2.1 AA audit of a vanilla composer create-project statamic/statamic install with no addons and no custom code. Tested with axe-core 4.x on Chromium 153 at 1440x1000, plus manual keyboard traversal. Source references point at v6.31.0.