test(fusion_accounting_reports): coexistence behavior

Mirrors Phase 1's coexistence test pattern. Verifies:

- The coexistence group (group_fusion_show_when_enterprise_absent)
  exists and is referenceable
- The reports engine model (fusion.report.engine) is always
  registered, regardless of Enterprise install state
- The Financial Reports root menu requires the coexistence group
- The Open Report... sub-menu (period picker wizard) is gated too

Uses V19 group_ids attribute with a graceful fallback to groups_id for
older runtime variants.

Tests: 3 new (test_coexistence.py). Net 115 -> 118.
Made-with: Cursor

fusion_accounting_reports/tests/__init__.py

@@ -22,3 +22,4 @@ from . import test_pdf_export
 from . import test_xlsx_export
 from . import test_period_picker
 from . import test_migration_round_trip
+from . import test_coexistence