Stop the agent's unit last when uninstalling it

[Philip Lombardi (plombardi89)]

Problem

When the daemon uninstalls its own unit, for an AgentReset MachineOperation or a node's deletion signal, UninstallService stops aks-flex-node-agent.service first. Stopping the unit sends the daemon SIGTERM, which cancels its context. The task then carries on:

• Removing the unit files still works, since it doesn't use the context.
• systemctl disable and systemctl daemon-reload fail with context canceled, and the task reports an error:

level=WARN msg="failed to disable service (may not be enabled)" unit=aks-flex-node-agent.service error="failed to start .../systemctl: context canceled"
level=ERROR msg=failed task=uninstall-service ... error="systemctl daemon-reload: failed to start .../systemctl: context canceled"

This leaves a dangling multi-user.target.wants/aks-flex-node-agent.service link, and systemd isn't reloaded. main's e2e doesn't catch it, because after an unjoin it only checks that the unit file is gone.

Fix

UninstallService now disables the unit, removes its files, and reloads systemd before it stops the unit, so nothing is left to run after the stop. systemd can still stop a running unit whose file is gone. A failed stop is still only a warning, and aks-flex-node reset still stops the daemon before it resets the node.

Testing

• Unit test: a new table-driven test checks the order of systemctl calls, that every file is gone before systemd reloads, and a stop that ends its caller. Moving the stop back to the front makes it fail.
• On a host: with this change, an AgentReset leaves no .wants link, the unit unloaded and inactive, and uninstall-service completing ok. Without it, it leaves the dangling link and the errors above. I checked this on an Azure Container Linux host running the agent from Run the agent as a privileged container on Azure Container Linux #370, which uses this same code path.
• Go tests and golangci-lint pass.

[Copilot]

🟢 Approval recommended

The focused ordering change addresses self-uninstallation failures, preserves warning-only stop handling, and includes regression coverage.

No open issues 🎉

What changed in this PR

Fixes agent self-uninstallation by completing systemd cleanup before stopping the daemon, avoiding context cancellation during cleanup.

Changes:

• Disables the unit, removes files, and reloads systemd before stopping.
• Adds regression coverage for ordering, cancellation, and failure handling.

File	Description
pkg/​daemon/​lifecycle.go	Moves service stop last and adds injectable test dependencies.
pkg/​daemon/​lifecycle_test.go	Tests cleanup ordering and stop/removal failures.

_{Review effort: Balanced}