refactor(java): replace JUnit assertions with AssertJ across all tests

[mmodzelewski]

Six test files still used JUnit assertions (assertEquals,
assertThrows, assertNotNull, etc.) instead of the project
standard AssertJ fluent API. This inconsistency made tests
harder to read and diverged from the convention used by
the rest of the test suite.

Migrated all JUnit assertion calls to AssertJ equivalents
and added an AssertJ note to the Contributing section in
README.md.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.26%. Comparing base (c90b5f8) to head (fc02a88).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2929      +/-   ##
============================================
- Coverage     70.28%   70.26%   -0.03%     
  Complexity      862      862              
============================================
  Files          1028     1028              
  Lines         85279    85279              
  Branches      62655    62655              
============================================
- Hits          59939    59921      -18     
- Misses        22825    22841      +16     
- Partials       2515     2517       +2     

Flag	Coverage Δ
java	59.87% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.
see 6 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.