Increase branch coverage from ~80% to 90% #71

Description

Increase overall branch coverage from the current ~80% baseline to at least 90%.

Acceptance criteria

• ./gradlew koverXmlReport shows overall branch coverage ≥ 90%
• ./gradlew test passes with no failures
• Kover verification rule in build.gradle.kts updated to minBound(90, CoverageUnit.BRANCH) and ./gradlew koverVerify passes

References

• Increase branch coverage from 67% to 90% #19 — predecessor, scoped to 75%