fix(schema): keep nested array manager when it shares its parent name

[theluckystrike]

Fixes #4129

When an array field holds a nested array with the same name, both register under the same scopedArrayName. The nested one replaced the parent's entry in arrayManagers, so getManagerOf on an outer item no longer knew its area field and rendering threw "has no field named area".

The comment above that code says the stored name keeps its flat form. So I left it alone, and nothing needs a migration. When a second array registers under a name that's already there, its fields are added to the existing manager's schema, and on a name clash the first one wins. The merged schema is a copy, so the parent field's own schema isn't touched.

Every reader I found looks fields up by name (area rendering, {% field %}, the wysiwyg list, the area patch path), and none validates or sanitizes through it.

test/arrays-nested-same-name.js renders an area on an outer item whose nested array shares its name, and checks that a stored item with the flat name still resolves its manager. Both fail on main. Both pass with the fix. The full mocha suite on sqlite gives the same results as main, apart from one test in document-versions.js that fails on both runs but not always the same one.

Michael