fix: allow singleton highlighter recovery after invalid language error

[Maxiemad]

• First call with invalid language throws (expected)
• Second call with valid language succeeds (fixed) Invalid lang on first codeToHtml call causes all future calls to fail #1089

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.24%. Comparing base (3c5d13e) to head (e4d7854).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1091   +/-   ##
=======================================
  Coverage   88.24%   88.24%           
=======================================
  Files          74       74           
  Lines        3291     3293    +2     
  Branches      945      945           
=======================================
+ Hits         2904     2906    +2     
  Misses        344      344           
  Partials       43       43           

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