fix(api): Don't block subscription events during WebSocket reconnect

[VarshithaPamisetty]

Description

The WebSocket reconnect ran on the serial event queue (asyncExpand), so a slow reconnect blocked every event behind it: subscription data and keep-alives were received but never processed, freezing active subscriptions on a healthy socket. This moves reconnect off the queue so events keep flowing, and guards against shutdown mid-reconnect.

Testing

• Regression test: data flows during an in-progress reconnect (fails without the change); plus a clean-shutdown-mid-reconnect test.
• WebSocket suite 35/35; analyze/format clean (Dart 3.11 and 3.13).
• Verified e2e against live AppSync: delivery, reconnect recovery, new subscription mid-reconnect.

Fixes #7001

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.98%. Comparing base (877bd59) to head (204d187).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7282   +/-   ##
=======================================
  Coverage   40.98%   40.98%           
=======================================
  Files         121      121           
  Lines        8273     8273           
  Branches     3598     3598           
=======================================
  Hits         3391     3391           
  Misses       4882     4882           

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