test: add DAG topology validation test suite

- Added comprehensive test suite for DAG topology validation module covering:
  - Adjacency map construction
  - Sink node identification
  - Topological sorting
  - Cycle detection
  - Structural validation
- Included test cases for common DAG patterns (linear, diamond, multiple sources)
- Added error case coverage for cycles, empty graphs, and isolated nodes
- Implemented detailed assertions to verify adjacency relationships and sort order

The tests ensure

Author: github-actions[bot]