fix: Update deprecated code editor test util documentation

[jperals]

Description

Updated test util doc to refer to onDelayedChange instead of deprecated onChange

Ticket: D461825569

How has this been tested?

Updated tests to cover onDelayedChange actually being called.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.43%. Comparing base (3d30633) to head (7344b41).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4573   +/-   ##
=======================================
  Coverage   97.43%   97.43%           
=======================================
  Files         941      941           
  Lines       29692    29692           
  Branches    10790    10791    +1     
=======================================
  Hits        28930    28930           
+ Misses        755      715   -40     
- Partials        7       47   +40     

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