fix(galaxy): bound alarm-subscription handles to one (no reconnect leak)

GalaxyDriver's StreamAlarms feed is session-less and survives an in-place
reconnect, so DriverInstanceActor re-subscribed on every Connected re-entry
(after dropping its own cached handle without an Unsubscribe — sync teardown).
The re-subscribe was additive: _alarmSubscriptions.Add grew the list by one
untracked handle per reconnect cycle — a slow unbounded leak. Functionally
harmless (the gate is Count>0 and OnAlarmFeedTransition only reads [0], firing
once regardless), but it accumulated forever.

Fix: SubscribeAlarmsAsync clears the set before adding, collapsing to a single
live handle (under the existing _alarmHandlersLock, atomic w.r.t. the fan-out
reader). There is exactly one consumer per driver instance (factory-per-actor
lifecycle), so replacing the set with the latest handle is faithful. Chosen
over making the actor's sync DetachAlarmSource call UnsubscribeAlarmsAsync
async/fire-and-forget — disproportionate for a minor leak.

Regression test Re_subscribe_collapses_to_a_single_handle_no_accumulation
(TDD-verified: FAILS without the Clear — releasing the latest handle leaves
the feed open because stale handles remain; PASSES with the fix). Galaxy tests
263 pass / 3 skip; Runtime native-alarm 24 pass. Code-reviewed (approved).

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverInstanceActor.cs

@@ -571,9 +571,9 @@ public sealed class DriverInstanceActor : ReceiveActor, IWithTimers
         // reconnects. NOTE: this does NOT tear down the driver-side subscription. For a session-bound
         // IAlarmSource the old subscription dies with the session (no accumulation). For a session-less feed
         // (GalaxyDriver's always-on central monitor) it survives an in-place reconnect, so the re-subscribe
-        // is additive — harmless because the gate only checks Count > 0 and the feed fans out once
-        // regardless of handle count, but it does slowly accumulate handles across many reconnects (a minor
-        // leak tracked as a follow-up; the correct cleanup is a driver-side reset on re-init).
+        // is additive — but the driver now collapses to a single live handle on each SubscribeAlarmsAsync
+        // (GalaxyDriver.SubscribeAlarmsAsync clears the set before adding), so handles no longer accumulate
+        // across reconnects. The gate (Count > 0) and the one-shot fan-out are unchanged.
         _alarmSubscriptionHandle = null;
     }