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.py

@@ -10,6 +10,8 @@ the bank-rec verification check. Phase 6 will add asset migration, etc.
 
 from odoo import _, api, fields, models
 
+from ..models.ir_module_module import GUARDED_MODULES
+
 
 class FusionMigrationWizard(models.TransientModel):
     _name = "fusion.migration.wizard"
@@ -34,7 +36,6 @@ class FusionMigrationWizard(models.TransientModel):
     @api.depends_context('uid')
     def _compute_detected(self):
         Mod = self.env['ir.module.module'].sudo()
-        from ..models.ir_module_module import GUARDED_MODULES
         installed = Mod.search([
             ('name', 'in', list(GUARDED_MODULES)),
             ('state', '=', 'installed'),