Add shared test fixtures and API-based data isolation

[choo121600]

Summary

Introduce a layered Playwright fixture system to replace fragile beforeAll browser-based setup, enabling reliable data isolation and deterministic cleanup.

This change lays the foundation for safe parallel E2E execution.

Why

E2E tests have been intermittently failing in CI, causing unreliable builds and frequent reruns, due to two root causes:

• UI-based data setup in beforeAll
  • Tests created seed data by opening a browser and clicking through the UI. This made setup fragile: dialog animations, rendering delays, and element timing issues could cause the setup itself to fail before any assertions ran.
• Insufficient data isolation between parallel workers
  • Test identifiers were generated with Date.now(), which can collide when multiple Playwright workers start at the same time. Leftover data from previous runs could also interfere with assertions that assumed exact row counts.

Core fixtures

• Add fixtures/pom.ts — POM fixtures with worker-scoped authenticatedRequest
• Add fixtures/data.ts — worker-scoped data fixtures (dagReady, successDagRun, executedDagRun, successAndFailedRuns) with try/finally cleanup

*https://playwright.dev/docs/test-fixtures
*https://playwright.dev/docs/test-parallel#isolate-test-data-between-parallel-workers

Domain-specific fixtures

• Add fixtures for asset, audit-log, calendar, dag-runs, dashboard, task-instances, and xcom

Utilities

• Add utils/test-helpers.ts — shared API utilities with UUID-based run IDs, 409 collision recovery, and idempotent cleanup

Test infrastructure

• Add global-teardown.ts — re-pauses DAGs after test run
• Update playwright.config.ts — globalTeardown

*https://playwright.dev/docs/api-testing#setup-and-teardown

This structure has been fully applied in PR #64024, where tests consistently pass with improved stability.

related: #63036

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