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/wizards/migration_wizard_views.xml

@@ -25,4 +25,22 @@
         <field name="view_mode">form</field>
         <field name="target">new</field>
     </record>
+
+    <!--
+        Top-level "Fusion Accounting" menu so the UserError guidance
+        ("Fusion Accounting -> Migrate from Enterprise") is actually reachable.
+        Placed at top level (no parent) because the migration is a one-time
+        admin task; making it visible during switchover is the point.
+        `groups` hides the menu from non-admins (mirroring the ACL on the wizard).
+    -->
+    <menuitem id="menu_fusion_migration_root"
+              name="Fusion Accounting"
+              sequence="95"
+              groups="fusion_accounting_core.group_fusion_accounting_admin"/>
+    <menuitem id="menu_fusion_migration_wizard"
+              name="Migrate from Enterprise"
+              parent="menu_fusion_migration_root"
+              action="action_fusion_migration_wizard"
+              sequence="10"
+              groups="fusion_accounting_core.group_fusion_accounting_admin"/>
 </odoo>