test(grafana): add comprehensive tests for API clients

[andreahlert]

Summary

Adds comprehensive unit tests for GrafanaApiClient and UnifiedAlertingGrafanaApiClient, which previously had zero test coverage.

33 new test cases organized in the following categories:

GrafanaApiClient (legacy alerting)

• Dashboard listing: tag-based search, query expression evaluation, URL qualification, folderTitle defaults, hyphenated tags
• Alert fetching: field mapping, URL construction with panelId/fullscreen params, dashboardTag parameter, empty results
• Error handling: 403 Forbidden, 500 Internal Server Error
• Proxy configuration: default /grafana/api path, custom proxy path
• Pagination: single page default, multi-page fetching, early stop on partial pages, upstream limit cap (5000), max pages safety limit

UnifiedAlertingGrafanaApiClient (unified alerting)

• Dashboard listing: delegates to shared search logic
• Alert fetching: single string selector, multiple selectors array, endpoint verification, no-match handling, rules without labels
• State aggregation: Alerting priority, Error priority, Pending priority, all-NoData, all-Normal, Normal+NoData mix, unrecognized states fallback to n/a, Alerting > Error priority ordering

Test plan

• All 33 new tests pass locally
• Full test suite (7 suites, 46 tests) passes with zero regressions
• ESLint passes on new test file
• Pre-commit hooks pass (eslint --fix, prettier --write)

[backstage-goalie]

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