Fix edit_constant to prevent class value mutation when not constant

[maximlt]

Fixes #1100

When a Parameter has already been instantiated (it is in the _param__private.params dict), we don't need to touch the class parameter.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.22%. Comparing base (5517f87) to head (57b2622).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1101   +/-   ##
=======================================
  Coverage   89.22%   89.22%           
=======================================
  Files           9        9           
  Lines        4715     4716    +1     
=======================================
+ Hits         4207     4208    +1     
  Misses        508      508           

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