Fix top-level fieldset error gutter

[BenjaminBeck]

Fixes #7799.

Summary

• add the missing horizontal gutter around top-level fieldset error borders in new and edit forms
• preserve the existing form DOM instead of adding a root .row
• keep the correction scoped to direct-child error fieldsets so it stops applying when tabs or columns wrap them
• rebuild the distributed CSS asset and cover the direct-child validation-error structure with a functional test

Rationale

Fieldsets inside tabs are children of a Bootstrap .row and receive its horizontal gutter. Top-level fieldsets are direct children of the form, so their .has-fieldset-error border otherwise sits immediately next to the field contents.

Wrapping the entire form in .row would introduce a layout regression when FormField::addColumn() is used. EasyAdmin already creates a row for the column group, which would result in .row > .row and doubled negative grid margins. Matching outer margin and inner padding on direct-child error fieldsets reproduces the gutter without changing the structure.

Tests

• FormLayout functional suite — 142 tests, 677 assertions
• full PHPUnit suite — 3,091 tests, 7,953 assertions
• PHPStan — no errors
• PHP CS Fixer dry run — no changes required
• logical CSS properties linter — no issues in 29 CSS files
• Biome — 44 files checked, no fixes required
• production asset build — successful
• browser verification at 1,440 px and 390 px — matching 12 px gutter and no horizontal overflow

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[BenjaminBeck]

CI note: the Symfony 8.0 job fails while loading fixtures because the test database is missing unrelated SQLite tables (sort_test_entity_many_to_many, blog_post, action_test_entity, and others). The same Symfony 8.0 job also fails on the exact 5.x base commit (88254355771dcb6829a9e5fea2812561b0046839): https://github.com/EasyCorp/EasyAdminBundle/actions/runs/32615613403/job/97135722391

All other required jobs for this PR pass, including PHPStan, all linters, PHP 8.2 with lowest dependencies, Symfony 6.4, Symfony 7.1 through 7.4, macOS, and Windows.