o/devicemgmtstate: add task to queue response messages

[st3v3nmw]

This PR implements the queue-mgmt-response task. Once a message has been applied (or rejected before reaching the subsystem), this task builds a signed response-message and queues it for transmission on the next store exchange.

For messages with a subsystem change, the task retries until the change completes, then calls the handler's BuildResponse to produce the response body and status. For messages that were rejected or not authorized before dispatch, the response is built directly from ResponseStatus & ResponseReason on the RequestMessage.

Split from #16248. Because the dispatch task is still in review, this PR shares sfrom code with #16547.

[codecov]

Codecov Report

❌ Patch coverage is 82.60870% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.51%. Comparing base (6d2e38e) to head (3e69d61).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
overlord/devicemgmtstate/devicemgmtmgr.go	82.60%	8 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #16656      +/-   ##
==========================================
- Coverage   77.57%   77.51%   -0.07%     
==========================================
  Files        1347     1353       +6     
  Lines      186281   186436     +155     
  Branches     2449     2449              
==========================================
+ Hits       144502   144509       +7     
- Misses      33044    33183     +139     
- Partials     8735     8744       +9     

Flag	Coverage Δ
unittests	77.51% <82.60%> (-0.07%)	⬇️

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[github-actions]

Wed Feb 25 15:14:36 UTC 2026
The following results are from: https://github.com/canonical/snapd/actions/runs/22390546512

Failures:

Executing:

• openstack:arch-linux-64:tests/main/interface-allow-auto-connection:unset
• openstack:ubuntu-core-20-64:tests/main/snap-pack
• openstack:ubuntu-25.10-64:tests/main/apparmor-prompting-flag-restart
• openstack:ubuntu-25.10-64:tests/main/prepare-image-components
• openstack:ubuntu-24.04-64:tests/unit/go:gcc

Restoring:

• openstack:ubuntu-25.10-64:tests/main/apparmor-prompting-flag-restart
• openstack:ubuntu-25.10-64:tests/main/
• openstack:ubuntu-25.10-64:
• openstack:ubuntu-25.10-64:tests/main/prepare-image-components
• openstack:ubuntu-25.10-64:tests/main/
• openstack:ubuntu-25.10-64:

Skipped tests from snapd-testing-skip

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