Fix hop-conf variable option handling

[gvdutra]

Summary

• report variable value and description updates as handled hop-conf actions
• add regression coverage for -sv and -dv

Root cause

The variable options updated the configuration but returned false from their handler. hop-conf therefore treated a successful update as if no action had been requested and displayed usage information.

Validation

• ./mvnw -pl engine -am -Dtest=SetHopConfigVariablesTest -Dsurefire.failIfNoSpecifiedTests=false test
• ./mvnw -pl engine -DskipTests spotless:check
• git diff --check

Closes #5979