fix: decouple remediation enhancer from private LLM helper

[LucaslfWang]

Summary

Fixes #167.

This PR removes the private _call_llm fallback import from the baseline remediation enhancer. LLM-based enhancement now relies only on an injected llm_fn callable, and gracefully returns None when no LLM callable is provided.

Added tests covering:

• enhancement using a provided llm_fn
• graceful fallback when no llm_fn is provided
• public enhancer behavior through enhance_generated_file

Type of Change

• Bug fix (non-breaking change fixing an issue)
• New feature (non-breaking change adding functionality)
• Breaking change (fix or feature causing existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Framework Changes Checklist

If this PR modifies the darnit framework (packages/darnit/):

• Updated framework spec (openspec/specs/framework-design/spec.md) if behavior changed
• Ran uv run python scripts/validate_sync.py --verbose and it passes
• Ran uv run python scripts/generate_docs.py and committed any doc changes

Control/TOML Changes Checklist

If this PR modifies controls or TOML configuration:

• Control metadata defined in TOML (not Python code)
• SARIF fields (description, severity, help_url) included where appropriate
• Ran validation to confirm TOML schema compliance

Testing

• Tests pass locally (uv run pytest tests/ -v)
• Added tests for new functionality (if applicable)
• Linting passes (uv run ruff check .)

Additional Notes

Local test run:

uv run pytest tests/darnit_baseline/remediation/test_enhancer.py