fix: render alerts for local-cluster monitors when data source is enabled (#1488)

[Aeternax]

Description

When data_source.enabled: true, viewing a local-cluster monitor's own Alerts tab — or clicking through from an alert on the Alerting overview into an individual alert — rendered a blank page: no alerts, no error, and no request to /api/alerting/alerts ever issued.

Root cause is in public/pages/Dashboard/containers/DashboardClassic.js, getAlerts():

if (dataSourceEnabled() && resolvedDataSourceId === undefined) {
  return;
}

For a plain local-cluster monitor (no data source association), resolvedDataSourceId is undefined, so this guard treated the normal local-cluster case as disqualifying and silently returned before making any request. Both the monitor's own Alerts tab and the individual-alert click-through render through this same method, so both went blank.

This change removes the early-return guard. params.dataSourceId is still only set when a data source actually resolves (a few lines further down), so the local-cluster case correctly omits it — and the server treats a missing dataSourceId as the local cluster. This mirrors the fix already applied to the overview alerts card in #1099 (DataSourceAlertsCard), bringing DashboardClassic in line with that accepted pattern.

A regression test (DashboardClassic.test.js) is added covering three cases: local-cluster + data source enabled (issues the request with no dataSourceId), data source enabled + resolved id (includes dataSourceId), and data source disabled.

Issues Resolved

Closes #1488

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.