[Test restructure 5] Refactor root conftest.py — move fixtures, remove marker filtering

[ntkathole]

What

The root tests/conftest.py currently does too much. Refactor it to match the new structure.

Changes

Move to universal/conftest.py:

• environment fixture and pytest_generate_tests parametrization logic
• universal_data_sources fixture (construct_universal_test_data)
• e2e_data_sources fixture
• feature_server_endpoint fixture
• feature_store_for_online_retrieval fixture
• vectordb_environment fixture
• fake_ingest_data, fake_document_data, fake_image_data

Delete from root:

• pytest_collection_modifyitems marker filtering logic (the entire function)
• --integration CLI option from pytest_addoption
• Related marker registration that only served filtering

Keep in root:

• simple_dataset_1, simple_dataset_2 (used across tiers)
• temp_dir, server_port (used across tiers)
• Marker registrations still needed for pytest --strict-markers
• --benchmark option and benchmark filtering (if still needed)

Validation

• pytest tests/unit runs without collecting any universal or integration tests
• pytest tests/universal still discovers and parametrizes the environment fixture correctly
• No fixture resolution errors across any tier

[GANESH-NADKARNI]

Hi! I noticed yashghatol's plan from June included this sub-issue (#6053) along with #6051, #6052, and #6054, but no PR ever materialized for it. I went ahead and opened a PR: #6804

While validating the change (per this issue's own "Validation" checklist), I found that removing pytest_collection_modifyitems exposed two mis-tiered test files that were previously silently protected by the marker-filtering logic:

• tests/unit/infra/compute_engines/flink/test_flink_compute_engine.py had one @pytest.mark.integration test hiding in unit/. Moved the whole file to tests/component/flink/, matching the existing precedent from [Test restructure 2] Move ray and spark tests to component/* #6050.
• tests/unit/test_rest_error_decorator.py — this one's actually already flagged in [Test restructure 4] Reclassify misplaced tests at the unit/integration boundary #6052's own investigation table as needing this fix. Moved it to tests/universal/ per that suggestion.

Happy to adjust anything or split these into separate PRs if that's preferred — let me know.