Add regression coverage for legacy alert rule payload normalization

## Problem
The current test suite does not cover legacy alert rule payload normalization.

## Expected behavior
- A focused Vitest case locks the existing behavior.
- The targeted test file passes and fails if the covered behavior regresses.

## Scope
Only add regression coverage for $(@{Slug=alert-rule-payload; Focus=legacy alert rule payload normalization; PrTitle=test(alert-rules): cover legacy payload normalization; TestFile=web/src/pages/ops/__tests__/alertRulePayload.test.ts; Mode=new; Append=/*
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import { describe, expect, it } from 'vitest';
import {
attachThresholdUnit,
normalizeDuration,
normalizeMetric,
thresholdUnits,
} from '../alertRulePayload';

describe('alert rule payload normalization', () => {
it('maps legacy Chinese metric labels to Prometheus metric names', () => {
expect(normalizeMetric('磁盘使用率')).toBe('rocketmq_disk_use_ratio');
expect(normalizeMetric('rocketmq_tps')).toBe('rocketmq_tps');
});

it('maps legacy Chinese duration labels to Prometheus durations', () => {
expect(normalizeDuration('1分钟')).toBe('1m');
expect(normalizeDuration('30分钟')).toBe('30m');
expect(normalizeDuration('45分钟')).toBe('45分钟');
});

it('attaches the metric threshold unit after normalization', () => {
expect(
attachThresholdUnit({ metric: '磁盘使用率', duration: '1分钟', threshold: 80 }),
).toEqual({
metric: 'rocketmq_disk_use_ratio',
duration: '1m',
threshold: 80,
thresholdUnit: '%',
});
expect(thresholdUnits['rocketmq_consumer_lag_messages']).toBe('条');
});
});}.TestFile). No runtime behavior changes.

## Acceptance criteria
- [ ] The targeted Vitest file passes.
- [ ] git diff --check is clean.

Start with web/src/pages/ops/__tests__/alertRulePayload.test.ts and the imported alertRulePayload module. Run the targeted Vitest file, add regression coverage for legacy metric and duration normalization plus threshold units, and confirm the test passes with git diff --check clean.

Written by the indexing model from the issue text.