o/devicemgmtstate: add task to apply messages

[st3v3nmw]

This PR implements the apply-mgmt-message task. This task hands off the message payload to the registered subsystem handler via handler.Apply, which creates a subsystem change and returns its ID. On success, the change ID is stored in the message so that queue-mgmt-response can later wait on it and build the response. On failure, the message is marked MessageStatusError rather than propagating the error as a task failure.. the pipeline continues to queue-mgmt-response which will send back the error status.

The task is a no-op when the message already carries an error from a prior stage (e.g., validation failure), letting it continue on to queue-mgmt-response.

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

[github-actions]

Wed Feb 25 14:48:50 UTC 2026
The following results are from: https://github.com/canonical/snapd/actions/runs/22389351425

Failures:

Executing:

• openstack:ubuntu-core-20-64:tests/core/kernel-base-gadget-single-reboot-failover
• openstack:ubuntu-25.10-64:tests/main/apparmor-prompting-integration-tests:create_multiple_deny
• openstack:ubuntu-24.04-64:tests/main/prepare-image-gating:local

Restoring:

• openstack:ubuntu-core-20-64:tests/core/kernel-base-gadget-single-reboot-failover
• openstack:ubuntu-core-20-64:tests/core/
• openstack:ubuntu-core-20-64:
• openstack:ubuntu-24.04-64:tests/main/prepare-image-gating:local
• openstack:ubuntu-24.04-64:tests/main/
• openstack:ubuntu-24.04-64:

Skipped tests from snapd-testing-skip

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