Fix ipywidgets Output context manager not working with MetaKernel

[blink1073]

Summary

• ipywidgets.Output's with output: context manager failed to route output to the widget when used with MetaKernel because get_ipython() returned None and comm.kernel was None at construction time
• Fixed by overriding comm.create_comm in MetaKernel.__init__ to pre-set kernel=self on newly created comms, enabling the comm.kernel fallback path in Output.__enter__ to call kernel.get_parent() and set msg_id correctly
• Added a regression test that simulates an execute request and verifies msg_id is set/cleared correctly when entering/exiting the context manager

Closes #217

Test plan

• just test passes (458 passed, 11 skipped)
• New test TestIpywidgetsOutputContextManager::test_output_widget_context_manager_sets_msg_id covers the regression

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.92%. Comparing base (cca4a2e) to head (e74d764).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
metakernel/_metakernel.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
- Coverage   90.95%   90.92%   -0.04%     
==========================================
  Files          51       51              
  Lines        2875     2886      +11     
  Branches      403      404       +1     
==========================================
+ Hits         2615     2624       +9     
- Misses        181      183       +2     
  Partials       79       79              

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