fix: Represent string literals as single-value unions

[jperals]

Issue #, if available:

Description of changes:

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.07%. Comparing base (2753437) to head (0344e1d).

❌ Your project check has failed because the head coverage (84.07%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (2753437) and HEAD (0344e1d). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (2753437)	HEAD (0344e1d)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #107       +/-   ##
===========================================
- Coverage   96.68%   84.07%   -12.61%     
===========================================
  Files          11       10        -1     
  Lines         845      471      -374     
  Branches      290      126      -164     
===========================================
- Hits          817      396      -421     
- Misses         28       75       +47     

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