test: add additional tests for failure modes

[mccutchen]

Plus, promote Hanshake() to the public API in support of those tests.

[github-actions]

🔥 Run benchmarks comparing 7a25e5f against main:

gh workflow run bench.yaml -f pr_number=36

Note: this comment will update with each new commit.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.43%. Comparing base (c0dcb9a) to head (7a25e5f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   83.82%   88.43%   +4.61%     
==========================================
  Files           3        3              
  Lines         445      441       -4     
==========================================
+ Hits          373      390      +17     
+ Misses         53       37      -16     
+ Partials       19       14       -5     

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