refactor: consolidate instance merging logic and add unit tests

[Tener]

I found myself adding yet another copy of discover...UserTaskAddExistingInstances for another PR I'm working on, so instead I extracted the mergeExistingInstances instead, replacing current 3 variant implementations.

I also fixed what I believe is incorrect merge of old and new data by making this conditional:

// Merge existing cluster state into in-memory object.
failedClusters.Clusters[existingClusterName] = existingCluster

like this:

// Merge existing cluster state into in-memory object, but only if we don't have a fresh key.
if _, found := freshInstances[instanceKey]; !found {
	freshInstances[instanceKey] = instance
}

Changelog: Fixed potential delay in updating User Task status for Discovery resources.

[Tener]

@nklaassen gentle ping.

[backport-bot-workflows]

@Tener See the table below for backport results.

Branch	Result
branch/v18	Create PR