test: add 66 unit tests for balance/quota notify + plan validation
balance_notify_service_test.go (27 tests):
- resolveBalanceThreshold: fixed/percentage/zero recharged/empty type
- quotaDim.resolvedThreshold: fixed normal/exceed/equal limit, percentage 0/30/100/>100, zero/negative limit
- sanitizeEmailHeader: CRLF/CR/LF/clean/empty/multiple newlines
- buildQuotaDims / buildQuotaDimsFromState: all dimensions, empty extra, state-vs-account precedence
- collectBalanceNotifyRecipients: empty, filter disabled/unverified, case-insensitive dedup, skip empty, trim
balance_notify_check_test.go (16 tests):
- CheckBalanceAfterDeduction guard clauses: nil user/disabled/global-off/threshold=0/user-override/no-crossing
- CheckAccountQuotaAfterIncrement guards: nil account/zero cost/negative cost/global-disabled
- getBalanceNotifyConfig: all fields, disabled, invalid threshold
- isAccountQuotaNotifyEnabled: missing/false/true
- getSiteName: default fallback + configured
balance_notify_email_body_test.go (10 tests):
- Guards against fmt.Sprintf arg-count mismatches in email templates
- Verifies HTML escaping of recharge URL
- Verifies CSS %% escape produces literal % in output
- Verifies unlimited/percentage/over-quota display branches
payment_config_plans_validation_test.go (13 tests):
- validatePlanRequired: all 5 validation branches + whitespace handling
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