E2E coverage for the trace search RED metrics view #3176

Context

PR #2826 replaces the single count histogram on trace search results with RED metrics (Throughput, Errors, Duration) and adds a RED/Heatmap switch. The aggregation builders and the chart color pipeline are unit tested, but nothing drives the view end to end.

The existing search e2e specs cover the histogram on a Logs source (Histogram drag-to-zoom preserves custom SELECT columns in packages/app/tests/e2e/features/search/search.spec.ts), and that path is unchanged: the RED view only renders for trace sources. So the new view currently has no e2e coverage.

What to cover

The e2e fixtures already seed an E2E Traces source and several specs switch to it via searchPage.selectSource(DEFAULT_TRACES_SOURCE_NAME), so this should fit the existing page-object pattern:

• Selecting a trace source renders the three RED tiles; a Logs source still renders the single histogram.
• The Errors tile switches between Rate and Volume.
• The RED/Heatmap switch swaps the area for the duration heatmap, and the mode survives a reload (it is stored in the URL as traceChartMode).
• Clicking a status series on the Errors volume chart filters the results.

Notes

The RED tiles render through ChartCard, which already accepts a data-testid, but the trace search tiles do not pass one today. Adding stable test ids for the three tiles is probably the first step so the spec does not have to match on visible title text.