Replaced direct create_engine usage with StaticPool-based connection
in resource_dag.robot to prevent SQLite connection sharing issues in
tests. Updated cycle detection test to use distinct resource types
(git-checkout and fs-directory) instead of the same type for both
resources, improving test coverage of cross-type cycle detection.
Split from PR #1204 per reviewer request for atomic commits.
ISSUES CLOSED: #1226
brent.edwards