[Python] Only auto re-subscribe after initial subscription

The subscription logic waits for the first successful subscription
before the Read() call is being returned (the future is awaited which
is only released on handleSubscriptionEstablished). If the first
subscription attempt fails (e.g. because the CASE session doesn't
establish) the Read() never returns, not with an error but also not
with a subscription transaction. And since the Python side has no
access to the SubscriptionTransaction object at this point yet,
there is also no way to stop this subscription attempt.

With this change, we only resubscribe if the initial subscription was
successful. This changes semantics slightly, but really allows the
caller to decide if it wants to continue try to establish the
subscription.

src/controller/python/chip/clusters/attribute.cpp

@@ -145,18 +145,20 @@ class ReadClientCallback : public ReadClient::Callback
 
     void OnSubscriptionEstablished(SubscriptionId aSubscriptionId) override
     {
+        // Only enable auto resubscribe if the subscription is established successfully.
+        mAutoResubscribeNeeded = mAutoResubscribe;
         gOnSubscriptionEstablishedCallback(mAppContext, aSubscriptionId);
     }
 
     CHIP_ERROR OnResubscriptionNeeded(ReadClient * apReadClient, CHIP_ERROR aTerminationCause) override
     {
-        if (mAutoResubscribe)
+        if (mAutoResubscribeNeeded)
         {
             ReturnErrorOnFailure(ReadClient::Callback::OnResubscriptionNeeded(apReadClient, aTerminationCause));
         }
         gOnResubscriptionAttemptedCallback(mAppContext, ToPyChipError(aTerminationCause),
                                            apReadClient->ComputeTimeTillNextSubscription());
-        if (mAutoResubscribe)
+        if (mAutoResubscribeNeeded)
         {
             return CHIP_NO_ERROR;
         }
@@ -243,6 +245,7 @@ class ReadClientCallback : public ReadClient::Callback
 
     std::unique_ptr<ReadClient> mReadClient;
     bool mAutoResubscribe = true;
+    bool mAutoResubscribeNeeded = false;
 };
 
 extern "C" {