fix(fusion_accounting_migration): add menu + tighten safety-guard test coverage

Addresses code review feedback on Task 17:
- Add menuitem so 'Fusion Accounting -> Migrate from Enterprise' is reachable
  (the UserError guidance now actually works). Placed at top level since
  parenting under fusion_accounting_ai.menu_fusion_accounting_root would
  require adding that module as a hard dep, which is wrong semantically
  (migration should not require AI). Both menuitems carry the admin group
  so the menu stays hidden from users who can't open the wizard anyway.
- Update the UserError wording to "Fusion Accounting -> Migrate from
  Enterprise" (no longer "Settings -> ...") to match the actual menu
  location; 'migration' is preserved per the test's assertIn check.
- Add skipTest guard to test_uninstall_not_blocked_when_migration_completed
  so it doesn't pass vacuously on Community-only CI (the guard's
  `if not installed: continue` would otherwise return True regardless of
  the flag value, giving a false green).
- Move GUARDED_MODULES import to top of wizards/migration_wizard.py
  (no circular-import risk -- models/ir_module_module.py doesn't import
  from wizards/).
- Expand docstrings on button_immediate_uninstall and module_uninstall
  overrides to note they may both fire in a single UI uninstall call
  and that the guard is idempotent (pure read + raise).

Made-with: Cursor

fusion_accounting_migration/tests/test_safety_guard.py

@@ -7,11 +7,22 @@ class TestSafetyGuard(TransactionCase):
     """Verify the safety guard blocks Enterprise uninstall when migration hasn't run."""
 
     def test_uninstall_not_blocked_when_migration_completed(self):
-        """If the per-module migration flag is set, uninstall is allowed."""
+        """If the per-module migration flag is set, uninstall is allowed.
+
+        Skip if account_accountant isn't installed -- otherwise the guard's
+        `if not installed: continue` short-circuit would make this test pass
+        vacuously without exercising the flag-check branch.
+        """
+        Module = self.env['ir.module.module'].sudo()
+        if not Module.search_count([
+            ('name', '=', 'account_accountant'),
+            ('state', '=', 'installed'),
+        ]):
+            self.skipTest("account_accountant not installed in this DB")
         self.env['ir.config_parameter'].sudo().set_param(
             'fusion_accounting.migration.account_accountant.completed', 'True'
         )
-        guard = self.env['ir.module.module']._fusion_check_uninstall_guard(['account_accountant'])
+        guard = Module._fusion_check_uninstall_guard(['account_accountant'])
         self.assertTrue(guard, "Guard should pass when migration flag is set")
 
     def test_uninstall_blocked_when_migration_pending(self):