Group fieldtype does not apply entry validation replacements ({id}, {collection}, {site}) #15513

Bug description

Class-based validation rules that use {id}, {collection}, or {site} placeholders receive null when the field lives inside a Group fieldtype. Saving the same entry then treats the current entry as a duplicate of itself (for uniqueness-style rules).

This is the same class of bug as #10065, which was fixed for Replicator and Grid in #10255 by adding AddsEntryValidationReplacements. Group was never given that trait.

Expected: nested class-based rules inside a Group get the same replacements EntriesController@update already passes at the top level (id, collection, site).

Actual: Group::extraRules() builds a fresh validator with only {this} prefix context. Unmatched placeholders become the literal null in Validator::parseClassBasedRule(), so e.g. new \Statamic\Rules\UniqueEntryValue({collection}, {id}, {site}) is constructed as new UniqueEntryValue(null, null, null).

Still present on master as of v6.34.0 (src/Fieldtypes/Group.php has no AddsEntryValidationReplacements; Grid and Replicator do).

How to reproduce

1. Fresh Statamic 6 site.
2. On any collection blueprint, add a Group with a nested text field that uses the documented unique-entry rule:

-
  handle: details
  field:
    type: group
    display: Details
    fields:
      -
        handle: code
        field:
          type: text
          display: Code
          validate:
            - 'new \Statamic\Rules\UniqueEntryValue({collection}, {id}, {site})'

3. Create an entry, set details.code to alpha, save.
4. Re-open that same entry, change any other field (or change nothing), save again.

Result: uniqueness validation fails against the entry being edited, because {id} was not substituted so the except argument is null.

The same null substitution happens for any new \Some\Rule({id}) inside a Group, not only UniqueEntryValue.

Why: Group::extraRules():

$rules = $this
    ->fields()
    ->addValues((array) $this->field->value())
    ->validator()
    ->withContext([
        'prefix' => $this->field->validationContext('prefix'),
    ])
    ->rules();

Grid/Replicator call addEntryValidationReplacements() on that nested validator. Group does not.

PR #11931 only fixed {this} prefixing for Group children inside Replicator/Grid; it did not pass {id} / {collection} / {site}.

Suggested fix: use AddsEntryValidationReplacements on Statamic\Fieldtypes\Group and call addEntryValidationReplacements($this->field, $validator) in extraRules(), matching Grid/Replicator.

Logs

No application exception. Validation error is the uniqueness / custom-rule failure with {id} unresolved.

Environment

Sanitized (no project URL, addons, or app name). Reproduced against Statamic v6.33.0; code still missing the trait on master / v6.34.0.

Environment
Laravel Version: 13.32.0
PHP Version: 8.4
Environment: local
Debug Mode: ENABLED

Statamic
Version: 6.33.0 PRO

Installation

Fresh statamic/statamic site via CLI

Additional details

Related: #10065, #10255, #11931.

Happy to open a PR that adds the trait to Group plus a test mirroring the Grid/Replicator replacement coverage, if that helps.