Fix template merge logic to preserve allOf blocks

[erral]

Discovered that merge_template_configs was dropping the allOf block from JSON schemas when merging a downstream template with an upstream one.
Since conditional questions are defined using allOf / if / then blocks, any conditions defined in a downstream template (custom project) were being lost.

This PR:

• Updates merge_template_configs to correctly append downstream allOf items to the upstream list.
• Adds a regression test in tests/config/test_merge_template.py.

This works in tandem with the fix in tui-forms (collective/tui-forms#23) to enable conditional questions in cookieplone templates.
Related tui-forms PR: collective/tui-forms#23

[ericof]

Just run the formatter and should be ready to be merged