#975 A dependency on a nonexistent task is accepted; dependent tasks are silently skipped and the run completes

EARS: If a workflow definition names a dependency that is not a task in the definition, then the Highway API shall reject the submission with a validation error naming the task and the missing dependency. EVIDENCE (#961): definition where 'code' depended on 'use_test' (absent after an edit); POST /api/v1/workflows accepted it, run 8f2d820c reported completed with only produce/use_plain/use_quoted executed; code, use_code and later tasks never ran and nothing reported it.