[WIP] zedmanager: fix purge stuck when app was never activated

[eriknordmark]

DO NOT REVIEW YET - TESTS IN PROGRESS

Description

There was a regression introduced in 16.9.0 from #5618 in that it did not take into account an application which has never started and is purged. We have a test case for that in the old ztests collection (test_app_purge_bad) but we do not seem to have such a case in the Eden tests.

This PR addresses that.

When a purge is triggered for an app that never started (e.g. its image failed to download), doUpdate transitions PurgeInprogress from DownloadAndVerify to BringDown and calls doRemove. Because no domain was ever created, doRemove returns done=true immediately. Without this fix the code fell through with PurgeInprogress still set to BringDown, skipped the RecreateVolumes check, and entered doPrepare/doActivate without ever calling purgeCmdDone. The BringDown state left in the published status then caused every subsequent updateAIStatusUUID call to route to removeAIStatus, which would tear down the prematurely created domain and leave the app oscillating indefinitely.

Fix by using BringDown only as an internal sentinel for doRemove/doInactivate (so it treats the operation as a purge rather than an uninstall). If doRemove completes immediately there is nothing to halt, so skip persisting BringDown and transition directly to RecreateVolumes, calling purgeCmdDone to save the purge counter and clean up stale volume refs. HALTING is now only recorded when a domain is genuinely being waited on.

How to test and validate this PR

Create an edge app with an image with a bad URL (or bad sha256).
Deploy on app instance using that edge app - it should fail due to a download or verify failure.
Then update the edge app to use an image with a working URL (and a matching sha256) and purge the instance to make it pick up that new version. That should make the app instance get to running/online state.

Changelog notes

None (fixes a regression introduced in 16.9.0 hence invisible to LTS users)

PR Backports

• 16.0-stable No
• 14.5-stable No
• 13.4-stable No

Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 26.56%. Comparing base (2281599) to head (423f47e).
⚠️ Report is 439 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5742      +/-   ##
==========================================
+ Coverage   19.52%   26.56%   +7.03%     
==========================================
  Files          19       24       +5     
  Lines        3021     4213    +1192     
==========================================
+ Hits          590     1119     +529     
- Misses       2310     2872     +562     
- Partials      121      222     +101     

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