Stabilize frontend CI tests by adding global ResizeObserver test setup

[Copilot]

The Studio Tests frontend job was still failing in CI due to ReferenceError: ResizeObserver is not defined under Vitest/jsdom. This surfaced in open-in-new-tab.test.tsx when components using responsive observers mounted during test execution.

• Root cause

  • Vitest runs in jsdom, which does not provide ResizeObserver by default.
  • Components like ReportViewer/ReportLinkModal instantiate ResizeObserver in effects, causing runtime failure before test assertions.

• Changes in this PR

  • Centralized test runtime shim
    • Added src/test/setup.ts to define a no-op ResizeObserver when absent in the test environment.
    • Stub matches expected API shape (observe, unobserve, disconnect) to avoid argument/signature drift.
  • Vitest configuration wiring
    • Updated vite.config.ts to load the shared setup via test.setupFiles.

• Why this shape

  • Keeps component code unchanged and browser-faithful.
  • Prevents per-test ad hoc mocks and ensures consistent behavior across the frontend test suite.

// vite.config.ts
test: {
  environment: "jsdom",
  setupFiles: "./src/test/setup.ts",
  passWithNoTests: true,
}