Add automated tests for investigation-scoped idempotent graph merges #39
Description
Problem
Fingerprint-based identity is now enforced at the types and repository layer,
but there is no automated test validating idempotency at the database merge level.
This makes it harder to detect regressions where duplicate nodes or relationships
could be reintroduced during future refactors.
Proposed Solution
Add an integration-style test that:
- Runs the same graph import twice for the same sketch_id
- Asserts node + relationship counts remain unchanged
- Confirms fingerprints remain stable across runs
Benefits
Ensures long-term correctness and protects against duplication regressions.
Please assign this to me under SWoC26 if it aligns with the Project's goals