Re-enable websocket ping/pong from the server

[Zsailer]

Looks like a minor regression was introduced by #1047

The server-side ping/pong on the kernel websockets as dropped when I refactored the websocket API.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (2d01f84) 80.37% compared to head (d7ed34b) 80.36%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1243      +/-   ##
==========================================
- Coverage   80.37%   80.36%   -0.01%     
==========================================
  Files          68       68              
  Lines        8263     8264       +1     
  Branches     1600     1600              
==========================================
  Hits         6641     6641              
  Misses       1200     1200              
- Partials      422      423       +1     

Impacted Files	Coverage Δ
jupyter_server/services/kernels/websocket.py	80.39% <100.00%> (+0.39%)	⬆️

... and 1 file with indirect coverage changes

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[blink1073]

Thanks!