[WIP] baseosmgr: gate EVE-k vs non-EVE-k upgrade block on volumes

[eriknordmark]

Description

The cross-flavor BaseOs upgrade between EVE-k and non-EVE-k flavors is
unconditionally rejected by baseosmgr with "Upgrade to EVE-k ... is
not supported" (or its inverse). The rationale is that the on-disk
/persist layout used by volumes differs across flavors, so existing
volumes can break across the switch. On a device with no volumes there
is nothing to disturb, so the block is overly strict.

This PR gates the check on volume presence:

• baseosmgr subscribes to VolumeConfig (from zedagent) and
  VolumeStatus (from volumemgr). When either set is non-empty, the
  cross-flavor mismatch errors out as before — the error now reads
  ... is not supported while volumes exist. With both sets empty the
  upgrade proceeds.
• A startup wait in baseosmgr blocks until both publications signal
  restart, so the first BaseOsConfig arrival after boot evaluates
  against the true volume set. Synchronized() is insufficient — it
  only confirms the socket-level handshake and can fire before zedagent
  has parsed its first EdgeDevConfig.
• volumemgr now calls pubVolumeStatus.SignalRestarted() at the tail
  of handleVolumeRestart. After zedagent restarts pubVolumeConfig,
  volumemgr has reconciled its on-disk recovery
  (populateExistingVolumesFormat*) with the controller-intended set
  into VolumeStatus, so subscribers have a Restarted() gate to
  reason about.
• Drive-by: pkg/pillar/types/diskmetrics_test.go used
  /persist/volumes/vol1 as a sample DiskPath; updated to
  /persist/vault/volumes/vol1 to match the canonical path
  (SealedDirName + "/volumes").

How to test and validate this PR

Same-flavor regression (covered by eden):

• eden test ./tests/update_eve_image -e update_eve_image_oci — PASS
  locally (12 min).
• eden test ./tests/update_eve_image -e update_eve_image_http — PASS
  locally (~10 min).
• Both verify the new baseosmgr startup wait does not block the upgrade
  flow and BaseOsConfig is processed across reboots.

Cross-flavor relaxation (manual):

• On a device with no app volumes, push a BaseOsConfig pointing at an
  EVE-k image from a running non-EVE-k image (or vice versa). The prior
  behavior errored out; the new behavior allows the upgrade.
• Deploy an app with a volume and retry the cross-flavor upgrade — the
  block returns with the new wording.

Changelog notes

• 16.0-stable: To be backported.
• 14.5-stable: No, this is a behavior change, not a bug fix.
• 13.4-stable: No, same reason.

Checklist

• I've provided a proper description
• I've added the proper documentation — operator-visible change is
  covered by the error-message text
• I've tested my PR on amd64 device — eden update tests on amd64
  qemu
• I've tested my PR on arm64 device — not tested
• I've written the test verification instructions
• I've set the proper labels to this PR — leaving to reviewers on
  mark-ready

[codecov]

Codecov Report

❌ Patch coverage is 0% with 51 lines in your changes missing coverage. Please review.
✅ Project coverage is 21.09%. Comparing base (324bd3c) to head (5b239d4).

Files with missing lines	Patch %	Lines
pkg/pillar/cmd/baseosmgr/baseosmgr.go	0.00%	44 Missing ⚠️
pkg/pillar/cmd/baseosmgr/handlebaseos.go	0.00%	5 Missing ⚠️
pkg/pillar/cmd/volumemgr/handlevolume.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5993      +/-   ##
==========================================
+ Coverage   20.68%   21.09%   +0.40%     
==========================================
  Files         491      501      +10     
  Lines       90599    92345    +1746     
==========================================
+ Hits        18744    19479     +735     
- Misses      70274    71105     +831     
- Partials     1581     1761     +180     

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[eriknordmark]

Superseded by #6036 (baseosmgr volume-gate + upgradeconverter ns port + volumemgr transitive blob walk, rebased onto master) and #6035 (diskmetrics test path fix). Closing this WIP.