o/snapstate: add secondary prerequisites task

[andrewphelpsj]

This secondary prerequisites task acts as the synchronization point, which ensures that a snap's prerequisites are available before the snap itself is installed. This second prerequisites task does not trigger any installations or refreshes.

This enables the earlier prerequisites task to trigger the installations and refreshes, which will allow seed-refresh to ensure that all prerequisites have been resolved prior to seed creation.

Note that this first PR does not actually resolve the issue with seed-refresh. Instead, it just introduces the new prerequisites task as the first local modification inducing task. Since the task itself is idempotent, there shouldn't be behavioral changes.

[codecov]

Codecov Report

❌ Patch coverage is 94.97487% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.13%. Comparing base (fcee949) to head (cb0a3f9).
⚠️ Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
overlord/snapstate/handlers_prereq.go	94.38%	5 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #17189      +/-   ##
==========================================
- Coverage   79.15%   79.13%   -0.02%     
==========================================
  Files        1376     1385       +9     
  Lines      192837   193288     +451     
  Branches     2466     2466              
==========================================
+ Hits       152640   152964     +324     
- Misses      31024    31144     +120     
- Partials     9173     9180       +7     

Flag	Coverage Δ
unittests	79.13% <94.97%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[github-actions]

Thu Jun 11 21:58:03 UTC 2026
The following results are from: https://github.com/canonical/snapd/actions/runs/27375343324

Failures:

Executing:

• openstack:ubuntu-24.04-64:tests/main/desktop-file-ids

Restoring:

• openstack:ubuntu-24.04-64:tests/main/nss-modules:winbind

Skipped tests from snapd-testing-skip

If you wish to have any of the below tests run in your PR, in your PR description, add 'unskip:' followed by a copy-and-pasted list (without variants) of the below tests you wish to run (unskip plus test list must be valid yaml)

• openstack:ubuntu-24.04-64:tests/main/i18n

[andrewphelpsj]

Here is a good graph that shows the inclusion of the new prerequisites task: