Improve Hydra test coverage to 100%

Improve Hydra's core test coverage incrementally.

The current coverage gate is 80%, and the core test suite currently reports less than complete coverage.

Goal

Reach and maintain 100% test coverage for Hydra core, then raise the enforced coverage threshold to 100%.

Coverage improvements should come from meaningful tests of supported behavior. Any code excluded from coverage should have a specific, documented reason.