Unit tests enhancement

[shining-cat]

This PR completes the unit test coverage improvements across all modules, achieving production-ready quality with 90.7% line and 76.4% branch coverage. All remaining gaps are infrastructure code (Kotlin stdlib, coroutines, framework wrappers).

Changes

Tests Added (9 new tests)

• Session Domain: UpdateUsersLastSessionTimestampUseCase - 8 comprehensive tests covering empty input, success, partial failure, error paths
• Session Domain: InsertSessionUseCase - 1 test for timestamp update failure scenario

Code Changes

• Session Shared-UI: Excluded SoundPoolFactoryImpl from coverage (framework wrapper with zero business logic)

Documentation

• Coverage Assessment: Updated to accept current coverage and shift focus to integration testing
  • Documented rationale for accepting 70-85% branch coverage (infrastructure gaps)
  • Added comprehensive integration testing roadmap (5 phases, 12-21 days)
  • Removed unit test improvement recommendations

Coverage Impact

Session Domain

• Class: 93.3% → 100% (+6.7%)
• Line: 86.1% → 97.4% (+11.3%)
• Branch: 72.8% → 82.6% (+9.8%) - Exceeds 80% target

Session Shared-UI

• Class: 92.3% → 96% (+3.7%)
• Line: 93.9% → 97.6% (+3.7%)

Overall Project

• Class: 94.5% → 95.3% (+0.8%)
• Line: 89% → 90.7% (+1.7%)
• Branch: 75.2% → 76.4% (+1.2%)

Testing Philosophy

✅ What we test: Business logic, error handling, edge cases, data transformations
❌ What we don't test: Kotlin stdlib (Flow, comparators), coroutines (withContext), Android frameworks (Room, Compose)

Result: Coverage metrics represent genuine protection of business logic, not inflated numbers from testing framework code.