Throw Error when non-public entity property is set via proxy

[roxblnfk]

🔍 What was changed

Throw Error when non-public entity property is set via proxy

🤔 Why?

The ORM does not write a value to a private property, nor does it report this. Now, the proxy will throw an Error when writing to a non-public property, which is similar to the standard behavior of PHP objects.

Ref: #499

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.60%. Comparing base (9703d14) to head (2cd6f7b).
Report is 3 commits behind head on 2.x.

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x     #502   +/-   ##
=========================================
  Coverage     91.59%   91.60%           
  Complexity     1814     1814           
=========================================
  Files           115      115           
  Lines          4642     4647    +5     
=========================================
+ Hits           4252     4257    +5     
  Misses          390      390           

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