core: implement SwapContext/Runtime for io.systemd.Unit.List + tests

core: implement SwapContext/Runtime for io.systemd.Unit.List + tests

Add varlink context and runtime builders for .swap units:

SwapContext: What, Priority, Options, TimeoutUSec, ExecActivate, ExecDeactivate
SwapRuntime: ControlPID, Result, CleanResult, UID, GID (SwapResult enum)

SwapResult is exposed as a proper varlink enum type. Runtime follows the same pattern as MountRuntime (ControlPID, Result, CleanResult, ref UID/GID).

The integration test is conditional since swap units may not be present on all systems.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>