test: align Kotlin SDK contract and assertion coverage

[josedab]

Summary

• Aligns the error contract and removes tautological assertions while documenting deferred stateful transport work.
• Preserves repository public contracts while restoring or strengthening the verification baseline.
• Includes an AUDIT.md where the repository reached the audit/refactor phase.

Commits

• 6298489 test(kotlin): align error contract coverage
• bf77ac3 docs(kotlin): record clean-code audit
• 144e42f test(kotlin): remove tautological assertions (KOTLIN-CC-1)

Validation

JDK 17 build and 279 tests pass without compiler warnings.

Known blockers / deferred work

• None beyond required PR checks and documented AUDIT.md deferrals.

Merge readiness

Ready for review. Merge only after all required GitHub checks and reviewer approvals pass.