Fix typo in parse plan exception

[johnpaulett]

Summary

• correct MSH typo in create_parse_plan
• test invalid first segment handling

Testing

• make lint
• make tests

---

https://chatgpt.com/codex/tasks/task_e_6845eb34d068832d84040d558438218e

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.38%. Comparing base (387d457) to head (dec81a4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
+ Coverage   90.24%   90.38%   +0.14%     
==========================================
  Files          20       20              
  Lines        1619     1623       +4     
  Branches      234      235       +1     
==========================================
+ Hits         1461     1467       +6     
+ Misses        112      111       -1     
+ Partials       46       45       -1     

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