[REF] Code quality improvements and structure fixes

- Add mypy.ini configuration to exclude migration scripts
- Rename migration files to proper snake_case (post-migration.py → post_migration.py)
- Add __init__.py to migration directories for proper Python package structure
- Add new portal access tests for website_sale_aplicoop
- Code formatting improvements (black, isort)
- Update copilot instructions and project configuration

Related to previous code quality refactoring work.

website_sale_aplicoop/tests/test_validations.py

@@ -349,7 +349,7 @@ class TestUserPartnerValidation(TransactionCase):
     def test_user_without_partner_cannot_access_order(self):
         """Test that user without partner_id has no access to orders."""
         start_date = datetime.now().date()
-        order = self.env["group.order"].create(
+        self.env["group.order"].create(
             {
                 "name": "Test Order",
                 "group_ids": [(6, 0, [self.group.id])],