Runs a flow once as a test, and accepts an unpublished (draft) flow — unlike
actions/trigger, which requires the flow to be published. Use this to verify a newly
authored or edited flow before publishing it.

Execution is identical to a production run: the same graph, the same bound connections,
the same credentials, and the same approval behaviour. An approval-gated node still parks
the run and its side effect still waits for a real approval — there is no request that
skips one.

The run is recorded as a test, so it does not appear in run history, does not count
towards dashboard statistics, does not become a dashboard widget's data source, and does
not consume a scheduled-run budget. It does hold the flow's concurrency lock, so a 409
is returned while the flow is already running.

The flow's first node must be a webhook, scheduled, or manual trigger.

Before dispatching, the flow is validated with the same checks publish applies — a draft
has never been through them. A flow that fails returns 422 listing every offending node
at once, so all of them can be fixed in one pass.

Poll the Location URL to follow the run and read what each node produced.

Pass ?dryRun=true to validate the flow without starting a run.