proxy: table preview admits schema-qualified targets the query gate rejects

[babltiga]

Problem

proxy/internal/DefaultSampleDataService.targetAllowed admits a table preview when
allowedTables.contains(bare) || allowedTables.contains(qualified) || allowedSchemas.contains(schema).

With allowed_tables = [orders], GET /api/v1/datasources/{id}/sample-rows?schema=archive&table=orders
therefore reads archive.orders, while the query gate
(workflow/internal/DatasourcePermissionChecker.rejectedTables → core.api.AllowedTables.coveringEntry)
rejects SELECT * FROM archive.orders. A bare entry only covers an unqualified reference in the gate;
the preview always addresses a concrete schema.table, so the bare-name shortcut widens it.

#936 (PR #1088) narrows introspectSchema for non-admins, which already hides the ambiguous case from
resolveTarget — but targetAllowed is the explicit second check and must stand on its own.

Fix

• Reuse core.api.AllowedTables (normalize / normalizeEntry / coveringEntry) in DefaultSampleDataService; delete its private normalizeList / normalize copies.
• A bare allowed_tables entry admits a schema-qualified preview target only when no other schema in the introspected view has a table of that name — the fail-closed rule core/internal/SchemaViewPermissionFilter uses (core: scope schema introspection to the caller's allow-list #936).
• Same dedup in DefaultQueryDryRunService (private normalizeList + inline matcher → AllowedTables).
• Unit tests in DefaultSampleDataServiceTest; document in docs/05-backend.md → "Sample data path (AF-443)".

Depends on #936 (PR #1088), which introduces core.api.AllowedTables.