HandleSubscribe spawned a Task.Run that mutated DataConnectionActor private
state (_subscriptionIds, _subscriptionsByInstance, _totalSubscribed,
_resolvedTags, _unresolvedTags) from a thread-pool thread, racing the actor's
own message loop — a data race on non-thread-safe Dictionary/HashSet and
non-atomic counters.
Restructured HandleSubscribe to follow the actor's existing PipeTo(Self)
pattern: the background task now performs only adapter I/O and pipes a
SubscribeCompleted message to Self; all subscription-state mutation happens
in the new HandleSubscribeCompleted handler on the actor thread (wired into
the Connected, Connecting and Reconnecting states).
Adds DCL001_ConcurrentSubscribes_DoNotCorruptSubscriptionCounters (30x30
concurrent subscribes) which fails against the pre-fix code and passes after.
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