feat(BA-6922): AppConfig fragment search REST v2 API (admin + scoped)

Fragment search surface, split out of the write PR (BA-6921, #12928):
system-wide admin search (superadmin) and principal-scoped search (auth),
with the filter/order/scope DTOs and the filter/order query converters.

Stacks on BA-6921 and reuses its AppConfigFragmentNode DTO. The scoped
search is the REST surface of the visible-fragments query (BA-6810).

Author: jopemachine

changes/12930.feature.md

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+Add the AppConfig fragment search REST v2 API: superadmin system-wide paginated search and principal-scoped fragment search (BEP-1052).