Improve test coverage with comprehensive E2E and unit tests

[google-labs-jules]

This PR significantly improves the test coverage of the repository by adding comprehensive End-to-End (E2E) tests for the advanced user workflow and unit tests for critical backend modules (export.py and pipelines.py). It also addresses flaky existing E2E tests by improving element selection and handling asynchronous UI updates more robustly.

Key Changes:

1. New E2E Test Suite: tests/e2e/test_workflow_completeness.py covers the full user journey from extraction to export, verifying advanced settings and system diagnostics.
2. Extended Unit Tests:
   • tests/test_core_export_extended.py: Tests core/export.py logic including cropping, dry runs, and filtering.
   • tests/test_pipelines_extended.py: Tests core/pipelines.py for edge cases like empty inputs, cancellation, and model initialization failures.
3. Fixes for Flaky Tests: Updated tests/e2e/conftest.py and tests/e2e/test_export_flow.py to use correct labels (e.g., "Use a Preset" instead of "Filter Presets") and robust wait conditions for Gradio's dynamic UI.
4. Documentation: Updated AGENTS.md and tests/TESTING.md to reflect the new tests and provide guidance on running them.
5. UI Tweak: Added title="Frame Extractor & Analyzer" to gr.Blocks in ui/app_ui.py to ensure the page title is correct for tests.

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PR created automatically by Jules for task 6026134942664270822 started by @tazztone

[google-labs-jules]

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