[codex] Fix quota notification threshold selection

[AdrianLeeElder]

Summary

• Send only the highest applicable base quota notification per billing pass
• Keep PAYG warning behavior unchanged
• Update billing background service tests for 80%, 90%, and exceeded threshold selection

Root cause

At 100% usage the billing job attempted the 80%, 90%, and 100% base notifications in the same pass, which could surface a misleading 90% subject even when usage was already at the quota.

Validation

• GRADLE_OPTS='-Xmx2g -XX:MaxMetaspaceSize=768m' ./gradlew --no-daemon :test --tests com.moneat.services.BillingBackgroundServiceTest -Dkotlin.compiler.execution.strategy=in-process
• git diff --check

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• Commit unit tests in branch codex/quota-notification-threshold

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☑️ Quality Gate passed

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Issues
✅ 0 New issues
ℹ️ 0 Accepted issues

Measures
✅ 0 Security Hotspots
✅ 100.0% Coverage on New Code
✅ 0.0% Duplication on New Code

See analysis details on SonarQube