* feat(balance): add volume state filter (ALL/ACTIVE/FULL)
Add a volume_state admin config field to the plugin worker volume balance
handler, matching the shell's -volumeBy flag. This allows filtering volumes
by state before balance detection:
- ALL (default): consider all volumes
- ACTIVE: only writable volumes below the size limit (FullnessRatio < 1.01)
- FULL: only read-only volumes above the size limit (FullnessRatio >= 1.01)
The 1.01 threshold mirrors the shell's thresholdVolumeSize constant.
* address PR review: use enum/select widget, switch-based filter, nil safety
- Change volume_state field from string/text to enum/select with
dropdown options (ALL, ACTIVE, FULL)
- Refactor filterMetricsByVolumeState to use switch with predicate
function for clearer extensibility
- Add nil-check guard to prevent panic on nil metric elements
- Add TestFilterMetricsByVolumeState_NilElement regression test
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