Await assembler task cancellations to prevent pending task error when program exits

[ChihweiLHBird]

Resolve #2564

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.720%. Comparing base (785d77f) to head (780d351).

Files with missing lines	Patch %	Lines
sanic/server/websockets/impl.py	55.555%	4 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #3157       +/-   ##
=============================================
- Coverage   87.793%   87.720%   -0.073%     
=============================================
  Files          105       105               
  Lines         8143      8152        +9     
  Branches      1290      1290               
=============================================
+ Hits          7149      7151        +2     
- Misses         687       695        +8     
+ Partials       307       306        -1     

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