Add equivalence relation tests for FluentBitSet.equals()

[lmj798]

Summary

Add boundary tests for FluentBitSet.equals() method to verify the
correct behavior of equivalence relation properties and edge cases
that are not covered by existing tests.

Why

Current tests cover basic equality and null comparisons, but do not
explicitly verify:

• Reflexivity: a BitSet equals itself
• Symmetry: if A.equals(B) then B.equals(A)
• Transitivity: if A.equals(B) and B.equals(C) then A.equals(C)
• Empty BitSet equality and hashCode consistency
• Clone equality and hashCode preservation
• Comparison with raw BitSet objects (should return false)
• Partial overlapping scenarios
• Same cardinality but different bit positions
  These boundary cases are easy to regress (for example, through
  off-by-one errors in bit comparison logic), so explicit tests
  improve behavioral stability.

Verification

Ran mvn -Dtest=FluentBitSetTest test
Result: all tests passed

[garydgregory]

@lmj798
This seems to duplicate existing tests in a test that has 100% code coverage:

• in the release, and
• in git master:

[lmj798]

@lmj798 This seems to duplicate existing tests in a test that has 100% code coverage:

• in the release, and
• in git master:

Ah, I see. I missed that those cases were already covered by the existing suite. Thanks for pointing that out. I'll close this PR.