cli/command/service: preserve mount order on force update

[kawmy]

- What I did

Fixed docker service update --force so it preserves the existing service mount order when no mount options are supplied.

This prevents a subsequent identical docker stack deploy from detecting a mount-order difference and unnecessarily rolling the service again.

Fixes #7226

- How I did it

Changed updateService() to call updateMounts() only when --mount-add or --mount-rm was explicitly changed.

Added a regression test that performs a force-only update and verifies that:

• ForceUpdate is incremented.
• The existing mount order remains unchanged.

- How to verify it

Run:

go test -mod=vendor ./cli/command/service -run '^TestUpdateServiceForcePreservesMountOrder$' -count=1
go test -mod=vendor ./cli/command/service -run '^TestUpdate' -count=1

The regression test, all service update tests, and the complete service package passed locally in a disposable container environment.

- Human readable description for the release notes

Preserve service mount order during forced updates to avoid an unnecessary rollout on the next stack deploy.