Improve checks on `_promote_charms` action

When passing a list of tags to promote, the workflow should make sure that all revision tags are present on the target commit. So no revision is left behind when promoting.

Trying to promote a revision that already exists on the target track is a no-op from charmcraft, so there is no problem on passing a complete list multiple times. Two different things are solved by checking this:
- If a failure happens mid promotion loop, we can pass the full list again. Any revision that was promoted will no-op and the loop will continue until the failure point and keep going.
- We avoid having an incomplete promotion being triggered by automated runs.

Extra reference: https://github.com/canonical/data-platform-workflows/pull/366#discussion_r3137318781