baseosmgr: add Phase-1 unit tests + pathConfig seam

[eriknordmark]

Description

Phase 1 of bringing pkg/pillar/cmd/baseosmgr/ from 0% to non-trivial
unit-test coverage. Companion to #5921 (the architecture doc).

The PR is split into two commits so the refactor can be reviewed
independently from the tests:

1. baseosmgr: introduce pathConfig seam for persistent files —
   refactor only; no behaviour change. Lifts the three on-disk paths
   baseosmgr persists across reboots
   (`current_retry_update_counter`, `config_retry_update_counter`,
   `forceFallbackCounter`) out of package-level constants and into
   a `pathConfig` struct hung off `baseOsMgrContext`.
   `defaultPathConfig()` returns the production paths under
   `/persist/`. The five callers (`saveCurrentRetryUpdateCounter`,
   `saveConfigRetryUpdateCounter`, `readSavedCurrentRetryUpdateCounter`,
   `readSavedConfigRetryUpdateCounter`, `readForceFallbackCounter`,
   `writeForceFallbackCounter`) now take `*baseOsMgrContext` and
   consult `ctxPtr.paths` instead of the constants.

2. baseosmgr: add Phase-1 unit tests — 10 new test files. No
   production change.

Coverage delta

Source	Before	After
go test -count=1 -cover ./cmd/baseosmgr/...	0.0%	16.1%

Phase-1 covers the pure helpers, lookups, publishes, content-tree
status, the persistent counter files, and the trivial dispatcher /
delete wrappers. The remaining ~70% of statements is the
zboot-driven state machine (`doBaseOsStatusUpdate`,
`doBaseOsActivate`, `validatePartition`, `validateAndAssignPartition`,
`handleZbootTestComplete`, `handleForceFallback`,
`updateBaseOsStatusOnReboot`, `handleUpdateRetryCounter`, the
install worker, `shouldDeferForNodeDrain`) plus `Run()` — all
unblocked by Phase-2's Zboot / kubeapi / HVType seams.

Test files added (one per logical concern):

• `helpers_test.go` — log init.
• `mocks_test.go` — `mockPubSub` (Publication + Subscription) and
  the `newTestCtx` fixture (per-test temp paths + all pubs/subs
  wired up).
• `baseosmgr_test.go` — `validateBaseOsConfig`, `appendError`,
  `publishBaseOSMgrStatus`.
• `lookups_test.go` — `lookupBaseOsConfig / Status /
  StatusByPartLabel / StatusesByContentID / ConfigByVersion`,
  `lookupContentTreeStatus`.
• `publish_test.go` — `publishBaseOsStatus`,
  `unpublishBaseOsStatus`, `publishZbootStatus`.
• `counters_test.go` — save / readSaved current and config retry
  update counters; absent / present / overwrite / garbage.
• `forcefallback_test.go` — read / write force-fallback counter;
  absent / present / overwrite / garbage.
• `contenttree_test.go` — `checkContentTreeStatus` missing /
  loaded-no-change / loaded-from-initial / error;
  `handleContentTreeStatusImpl` no-fanout.
• `globalconfig_test.go` — `handleGlobalConfigImpl` / `Delete`
  non-global key fast paths.
• `wrappers_test.go` — trivial Delete dispatch wrappers,
  `maybeRetryInstall` non-TooEarly / no-config branches,
  `shouldDeferForNodeDrain` NOTSUPPORTED short-circuit,
  `handleNodeDrainStatusImpl` non-UPDATE ignore.

Why draft

Drafted because the Phase-2 refactor + tests (Zboot/kubeapi/HVType
seams) is the larger, more reviewable chunk and may inform feedback
on the layering used here. Happy to mark ready for review once
Phase 2 has been seen, or sooner if reviewers want to look now.

PR dependencies

None hard. Reads better after #5921.

How to test and validate this PR

```sh
cd pkg/pillar
go test -count=1 -race -cover ./cmd/baseosmgr/...
go test -count=1 -coverprofile=/tmp/b.cov ./cmd/baseosmgr/...
go tool cover -func=/tmp/b.cov
```

Expected: all tests pass, coverage ≈ 16.1%.

Changelog notes

No user-facing changes.

PR Backports

• 16.0-stable: No, test-only addition.
• 14.5-stable: No, test-only addition.
• 13.4-stable: No, test-only addition.

Checklist

• I've provided a proper description
• I've added the proper documentation
• I've tested my PR on amd64 device — N/A, host-side unit tests
• I've tested my PR on arm64 device — N/A, host-side unit tests
• I've written the test verification instructions
• I've set the proper labels to this PR

[codecov]

Codecov Report

❌ Patch coverage is 51.85185% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 22.28%. Comparing base (63fa63d) to head (f71bebd).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/pillar/cmd/baseosmgr/paths.go	0.00%	5 Missing ⚠️
pkg/pillar/cmd/baseosmgr/baseosmgr.go	0.00%	3 Missing ⚠️
pkg/pillar/cmd/baseosmgr/forcefallback.go	57.14%	3 Missing ⚠️
pkg/pillar/cmd/baseosmgr/handlebaseos.go	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5922      +/-   ##
==========================================
+ Coverage   21.62%   22.28%   +0.66%     
==========================================
  Files         464      475      +11     
  Lines       83994    85700    +1706     
==========================================
+ Hits        18166    19101     +935     
- Misses      64300    64885     +585     
- Partials     1528     1714     +186     

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