chore: fix mypy warnings in tests/cs/prompts

What does this merge request do and why?

This MR fixes all mypy warnings on tests/code_suggestions/prompts/ folder.

❯ poetry run mypy tests/code_suggestions/prompts/
tests/code_suggestions/prompts/parsers/test_tree_traversal.py:100: error: Argument 1 to "append" of "list" has incompatible type "Node"; expected "type"  [arg-type]
tests/code_suggestions/prompts/parsers/test_counters.py:493: error: "dict" expects 2 type arguments, but 1 given  [type-arg]
tests/code_suggestions/prompts/parsers/test_blocks.py:156: error: Item "None" of "Node | None" has no attribute "type"  [union-attr]
tests/code_suggestions/prompts/parsers/test_blocks.py:157: error: Item "None" of "Node | None" has no attribute "start_point"  [union-attr]
tests/code_suggestions/prompts/parsers/test_blocks.py:157: error: Item "None" of "Node | None" has no attribute "end_point"  [union-attr]
Found 5 errors in 3 files (checked 8 source files)

How to set up and validate locally

1. Check out to this merge request's branch.
2. Ensure the following commands do not result in any warnings (they are already run as part of CI).

   make check-mypy
   make test

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.

Contributes to #715 (closed)