feat: paginate alert rules

[tju-yxq]

Closes #2589

Summary

• add GET /api/alert-rules/page with name search, enabled filter, bounded page, and bounded page size
• apply filtering and deterministic name ASC, id ASC ordering in SQL through MyBatis-Plus Page
• reject invalid page/pageSize before repository access
• preserve the unpaginated endpoint for export and existing callers
• replace full-inventory reads in toggleRule with direct findRuleById
• resolve bulk-toggle IDs with an ID-bounded findRulesByIds query
• wire the Alert Rules page to server-driven pagination, search, enabled filtering, server total, and stale-response protection
• add mock-mode support for the same page/filter contract

Why

The page previously fetched every rule and rendered it with pagination={false}. Toggling one rule also scanned the complete inventory, and bulk toggle read all rules to resolve a small ID list. This makes the read path bounded by page size and makes targeted mutations use targeted lookups.

Tests

• focused backend: AlertServiceTest, AlertRuleControllerTest, MybatisPlusAlertRepositoryTest — 80 tests passed, Checkstyle 0 violations
• focused frontend: AlertsPage.test.tsx, opsService.test.ts, ops.test.ts — 3 files / 32 tests passed
• backend full suite: 1,632 tests run; only the 2 pre-existing RocketMQMessageProviderTest failures remain, reproduced earlier on untouched upstream 14da4b95
• npm run lint -- --quiet: 0 errors, 2 pre-existing warnings
• npm run build: passed
• git diff --check: passed

Production changes: 176 additions / 48 deletions across backend and frontend, naturally exceeding 100 production lines without test padding.

Note

I also checked open PR #2533. Its later commits already contain a broader native-alerting implementation with alert-rule pagination, but that PR is still open and has been rebased several times. This PR is intentionally scoped to the current upstream code path, avoids the unrelated native-alerting domain changes, and provides a smaller standalone fix. If maintainers prefer to land #2533 first, this PR can be rebased or closed as superseded.

[RockteMQ-AI]

LGTM — code changes look good.

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