test(01-03): create test module structure for GNN

- Add tests/test_gnn/ directory
- Add __init__.py with module docstring
- Prepare for graph builder and model tests

Author: vmehtacode

tests/test_gnn/__init__.py

@@ -0,0 +1,12 @@
+"""Tests for the GNN verifier module.
+
+This module tests:
+- GridGraphBuilder: SSEN metadata to PyG Data transformation
+- TemporalEncoder: Time-window feature encoding
+- GATVerifier: Graph-based anomaly scoring
+
+Test categories:
+- Unit tests: Individual component correctness
+- Integration tests: End-to-end pipeline validation
+- Performance tests: Latency and accuracy benchmarks
+"""