cleanup(pubsub): remove MessageStream::close()

[dbolduc]

Fixes #5024

We can use the public APIs in our unit tests now and remove the test-only helper.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.98%. Comparing base (c166879) to head (aa6d436).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5263      +/-   ##
==========================================
- Coverage   97.98%   97.98%   -0.01%     
==========================================
  Files         215      215              
  Lines       44904    44899       -5     
==========================================
- Hits        43999    43994       -5     
  Misses        905      905              

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