`stage` comes from the shared schema, so this is contract tests only.
The test that matters asserts a *wrongly typed* stage is rejected rather than
only that a valid one is accepted. `PrinterTelemetrySchema` permits unknown
properties - verified empirically, `{ totally_bogus: 1 }` validates - so
`{ stage: 54 }` passes whether or not the field is declared, and a test built
only on the positive case would keep passing if someone deleted it.
Worth knowing separately: that open `additionalProperties` means a gateway
sending a misspelled field name is accepted and the value silently dropped,
which is the same drift this suite exists to catch. Not changed here, because
tightening it needs a check against what real gateways send.