[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.

product_sale_price_from_pricelist/migrations/18.0.2.1.0/__init__.py

@@ -0,0 +1,4 @@
+"""Make migrations folder a package so mypy maps module names correctly.
+
+Empty on purpose.
+"""

product_sale_price_from_pricelist/models/res_config.py

@@ -1,4 +1,3 @@
-from odoo import api
 from odoo import fields
 from odoo import models