[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