Fix road generation success when no RoadData is produced

[ErikSteiner]

Summary

Fixes a generation-status inconsistency in RoadGenerationService.generateTask().

Road.generateRoad() can legitimately return null, but generateTask() previously discarded that return value and reported success as long as no exception was thrown. safeGenerate() could therefore mark a StructureConnection as COMPLETED even though no RoadData had been produced.

Changes

• Treat Road.generateRoad() == null as an unsuccessful generation attempt.
• Add a regression test that deterministically exercises the real null path.
• Add JUnit Jupiter and Mockito as test-only dependencies for the common module.

The production change is intentionally minimal:

return new Road(level, conn, cfg, genCfg)
        .generateRoad(modCfg.pathfindingCost().aStarMaxSteps()) != null;

Existing exception handling and the existing behavior when roads are globally disabled are unchanged.

Testing

• Regression test demonstrated the previous bug first (expected false but was true).
• Updated regression test passes and explicitly verifies that the test setup makes Road.generateRoad() return null.
• :common:test --rerun-tasks: BUILD SUCCESSFUL (1/1 tests).
• git diff --check: clean.
• Built the Fabric mod from this branch and loaded it successfully in Minecraft 1.20.1.
• A fresh world could be created without an observed regression.
• The specific generateRoad() == null condition was not observed during the manual smoke test.

Scope

This PR only addresses the incorrect success contract around Road.generateRoad().

It does not change:

• the separate case where RoadData is persisted and a later precomputation step fails

Development tracking:
ErikSteiner#1