test: add coverage for asttools helpers - #365

Summary

• Add direct unit-test coverage for the AST helper query, value, and tool-node functions.
• Keep the existing file-editing tests while covering the remaining public helpers.

Testing

• uv run --with pytest --with astor --with asttokens pytest tests/test_generation_asttools.py
• uv run --with ruff ruff check tests/test_generation_asttools.py
• git diff --check

Closes #108