Safely allow migrate if in period when watcher’s running

[michalpristas]

Agent should not run migration when in upgrade in process or watching state.

Action should be declined and error visible in UI.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[michalpristas]

@nimarezainia @cmacknz should this be disabled in UI as well or is it enough when we fail with error agent side.
i'd rather not rely on ui to have clear picture of agent state. it can be wrong or delayed

[cmacknz]

If we get into this state, particularly if there are agents that refuse to migrate because they are stuck upgrading for whatever reason, we are going to need some guide in the UI for why the migration hasn't happened yet.

It's hard for me to view a nice solution to this that doesn't involve the UI at least being aware of this problem in some way.

If we fail the migrate action with an acknowledgement error on the agent, and that is in a discoverable place, that might be enough. The user questions I anticipate when using this feature will be things like:

1. How many of my agents have migrated?
2. Is the migration progressing or stuck?
3. Why is the migration stuck?
4. Did the migration fail? Why?

[ebeahan]

closing because after team discussion, we've decided to allow migrations during upgrades.