[Studio] Refresh alert rule threshold unit

[Aias00]

Summary

• derive alert rule thresholdUnit through one shared payload helper
• refresh thresholdUnit on edit as well as create so changing the metric cannot submit a stale unit
• add regression coverage for metric-to-unit payload mapping

Verification

• npm --prefix web test -- --run src/pages/ops/tests/alerts.test.ts
• (cd web && npm exec eslint -- src/pages/ops/alerts.tsx src/pages/ops/alertRulePayload.ts src/pages/ops/tests/alerts.test.ts)
• npm --prefix web run build

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[Aias00]

Superseded by #632. Folded into the consolidated ops request validation PR to keep alert request payload handling with the related ops validation changes.