Check if snap channel exist before provisioning TF machine

Prevent wasting time by checking if the snap channel exists, before starting the long running testflinger tests.

Example: https://github.com/canonical/inference-snaps-testing/actions/runs/25663234273/job/75377143686

Start with the GitHub Actions workflow associated with the linked example run and locate the step that provisions the Testflinger machine. Add a preflight check for the requested snap channel so unavailable channels stop the workflow before long-running tests begin, then verify the workflow's behavior for an existing and a missing channel.

Written by the indexing model from the issue text.