fix: clear stale transform rows immediately on error state

[Copilot]

TransformsPage showed stale rows for one render cycle after a refresh transitioned from success to error. The useMemo for transforms fell through to lastSuccessfulTransforms on error, and the useEffect clearing that state only runs after render.

• Add explicit result.status === "error" branch in the useMemo to return [] synchronously, rather than relying on the post-render effect:

const transforms = useMemo(
  () =>
    result.status === "success"
      ? result.data
      : result.status === "error"
        ? []
        : lastSuccessfulTransforms,
  [result, lastSuccessfulTransforms],
);

Original prompt

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This section details on the original issue you should resolve

<issue_title>[branch-actions-detective] CI main failure: TransformsPage stale-row regression breaks unit coverage</issue_title>
<issue_description>## CI Failure Summary

Workflow: CI
Branch: main
Run: https://github.com/elastic/ai-github-actions-playground/actions/runs/22924593115

Root Cause

The Unit Tests job failed in step "Unit tests with coverage (full)" because tests/component/TransformsPage.test.tsx failed at expect(screen.queryByText("tx-stale")).not.toBeInTheDocument().

The failure shows stale transform row tx-stale remains rendered after a refresh transitions from success to error ("Boom after refresh"), violating the intended behavior covered by the test.

Impact

This fails required CI on main and can block merges while the test remains red. It also indicates a user-visible state-management regression on the Transforms page (stale data shown during error state).

Suggested Fix

1. In TransformsPage data/render logic, clear or gate previously successful rows when the latest refresh is in error state.
2. Ensure table rendering is conditioned on current successful data (or explicitly reset rows on error) so stale rows are not shown.
3. Re-run make test-unit-coverage (or the specific vitest target) to confirm the failure is resolved.

Evidence

• Failing job: Unit Tests (job 66531643811), step: Unit tests with coverage (full).
• Log excerpt:
  • FAIL tests/component/TransformsPage.test.tsx > TransformsPage > does not keep stale rows visible after transitioning from success to error
  • Error: expect(element).not.toBeInTheDocument()
  • expected document not to contain element ... tx-stale
  • tests/component/TransformsPage.test.tsx:282:48
• Test assertion location in repo: peek/tests/component/TransformsPage.test.tsx:282.
• Related prior context: similar bug was previously fixed/closed in [performance-sweeper] Transforms page keeps stale rows visible after refresh error #2051, suggesting a recurrence/regression.

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• expires on Mar 17, 2026, 9:26 PM UTC

Comments on the Issue (you are @copilot in this section)

• Fixes [branch-actions-detective] CI main failure: TransformsPage stale-row regression breaks unit coverage #2350

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