chore: cleanup tests, remove unused method, move urlEncode method

[jimmyjames]

Sync changes from generator to SDK for test cleanup and removal of unused method.

Description

• cleanup tests and use assertJ (chore(java-sdk): cleanup tests sdk-generator#525) Thanks @piotrooo!
• move urlEncode to StringUtil, deprecate it in ApiClient (chore(java-sdk): cleanup tests sdk-generator#525) Thanks @piotrooo!
• remove unused private method ApiClient#valueToString (removed unused function valueToString sdk-generator#541) Thanks @AlessandroMinoccheri!

References

• removed unused function valueToString sdk-generator#541
• chore(java-sdk): Remove unused imports sdk-generator#543
• chore(java-sdk): cleanup tests sdk-generator#525

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 33.42%. Comparing base (1af5cc1) to head (68b531a).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #165   +/-   ##
=========================================
  Coverage     33.42%   33.42%           
- Complexity      991      994    +3     
=========================================
  Files           182      182           
  Lines          6902     6878   -24     
  Branches        771      772    +1     
=========================================
- Hits           2307     2299    -8     
+ Misses         4488     4476   -12     
+ Partials        107      103    -4     

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