dispatch: collapse mirror test helpers to a raw-JSON writer

Match the attach_test style: instead of constructing an EntireSettings struct and marshaling it, writeV1CustomRefMirrorSettings now writes the settings.json literal directly via testutil.WriteFile. Folds the mirrorEnabledSettings and enableV1CustomRefMirror helpers into the single writer and drops the now-unused encoding/json and settings imports.

Test-only cleanup; no behavior change.