Return $this instead of throwing "already assigned" exception in Manager::assign()

[arogachev]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	✔️

[what-the-diff]

PR Summary

• Enhancement to Manager::assign() Method
  An improvement has been added in the Manager::assign() method in our system. Now, rather than stopping the execution and throwing an "already assigned" exception, it simply gives back the control this allows for smoother operation.

• Unit Testing Updates
  As the assign() method behavior has been upgraded, we also needed to change the way we test it. Our tests have been modified to ensure this new behavior gives correct results under different conditions. Characteristics related to assigning already assigned items, instances when direct permission assignment is both enabled and disabled, and data persistency, have been updated in the test code.

• Code Readability Improvement
  As an added bonus, this PR also enhances readability of our test code. It introduces the use of named arguments when calling the assign() method, making it easier to understand what each argument represents.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a9a6712) 100.00% compared to head (ee874f9) 100.00%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##              master      #253   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity       217       217           
===========================================
  Files             13        13           
  Lines            546       544    -2     
===========================================
- Hits             546       544    -2     

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