DM-53212: Improve type annotations for sequence_to_string

[mfisherlevine]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.59%. Comparing base (d161c1f) to head (1b61df2).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #238   +/-   ##
=======================================
  Coverage   94.59%   94.59%           
=======================================
  Files          51       51           
  Lines        3883     3883           
=======================================
  Hits         3673     3673           
  Misses        210      210           

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