Vectorize get_fixed_string

[MichaelChirico]

#3102 re-uses this, and in both sites, encodeString() wraps the output.

I wanted to absorb this encodeString() call to the helper, and found the natural way to do so to vectorize the loop (an alternative is to split into two helpers, one to get a fixed regex from a single string, the other wrapping that).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ee2c75b) to head (5fecbe8).

Additional details and impacted files

@@                    Coverage Diff                     @@
##           string-boundary-metadata     #3103   +/-   ##
==========================================================
  Coverage                    100.00%   100.00%           
==========================================================
  Files                           129       129           
  Lines                          7487      7483    -4     
==========================================================
- Hits                           7487      7483    -4     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.