Fix condition evaluation and user_answers when root_key is used

[erral]

Found a bug in Form.is_active and Form.user_answers where they were not aware of root_key nesting.
When a root_key is defined (like cookiecutter in cookieplone), all answers are nested under that key.
The engine was looking for answers at the top level, causing all conditions to fail and user_answers to be empty or raise KeyError.

This PR:

• Makes is_active look inside root_key for gating answers.
• Makes user_answers retrieve values from the nested dict.
• Updates BaseRenderer.render to automatically record un-nested initial_answers under the root_key if one exists.
• Adds regression tests in tests/form/test_form_root_key.py.

This is required for conditional questions in cookieplone to work properly.