refactor: modularize codebase and add 404 unit tests

Split large files into focused modules across all crates while
maintaining API compatibility via re-exports. Add comprehensive
unit tests covering core models, pipeline parsers, LLM triage,
DAST security tools, graph algorithms, and MCP parameter validation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

compliance-agent/tests/common/mod.rs

@@ -0,0 +1,3 @@
+// Shared test helpers for compliance-agent integration tests.
+//
+// Add database mocks, fixtures, and test utilities here.