[Studio] Validate cluster flush disk type

[Aias00]

Summary

• convert invalid flushDiskType values into a 400 BusinessException instead of an unchecked enum parsing failure
• keep invalid config updates from reaching repository persistence
• add ClusterService regression coverage

Verification

• JAVA_HOME=$(/usr/libexec/java_home -v 21) mvn -f server/pom.xml -Dtest=ClusterServiceTest test

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[Aias00]

Superseded by #646. Consolidated with related cluster config and store loading changes to reduce review overhead while preserving the same scoped fixes and tests.