Fix flaky E2E tests with API-based setup and data isolation

[choo121600]

Problem

E2E tests relied on UI interactions to create test data, which led to frequent flaky failures due to:

• Timing issues (e.g. waiting for UI rendering / network idle)
• Data interference between parallel test runs

Change

• Replaced UI-based test data setup with direct API calls, removing timing-related instability in the setup phase
• Introduced shared utilities (test-helpers.ts) so each test creates and cleans up its own isolated data
• Added Playwright custom fixtures (fixtures.ts) to auto-inject Page Object instances, eliminating repetitive beforeEach boilerplate

Result

• Stable parallel execution by eliminating cross-test data interference
• Significantly reduced flaky failures across the test suite
• Improved readability and maintainability with consistent setup patterns across all 37 E2E test files

related: #63036

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  claude

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