test: refactor root conftest.py — move fixtures to universal/, remove marker filtering (#6053) - #6804

Closes #6053.

Moves the fixtures listed in the issue (environment, vectordb_environment, universal_data_sources, e2e_data_sources, feature_server_endpoint, feature_store_for_online_retrieval, fake_ingest_data, fake_document_data, fake_image_data) plus pytest_generate_tests and their supporting helpers (_load_universal_feature_repo_deps, start_test_local_server) into a new tests/universal/conftest.py, since they're hard dependencies of the fixtures being moved.

Deletes pytest_collection_modifyitems and the --integration CLI option from root conftest.py, per the issue.

Keeps simple_dataset_1/2, temp_dir, server_port, feature_store, auth_config, tls_mode, and marker registrations in root, as specified.

Additional fixes found during validation:

Removing the marker-filtering logic exposed two mis-tiered files that were previously silently protected by it:

• tests/unit/infra/compute_engines/flink/test_flink_compute_engine.py contained one @pytest.mark.integration test whose presence in tests/unit was masked by the old filtering. Moved the whole file to tests/component/flink/, matching the existing precedent from [Test restructure 2] Move ray and spark tests to component/* #6050 (component/ray, component/spark) — no test content changed.
• tests/unit/test_rest_error_decorator.py uses the environment fixture with no markers — this file is explicitly named in [Test restructure 4] Reclassify misplaced tests at the unit/integration boundary #6052's own investigation table as needing this exact fix ("move to universal/" was one of the two suggested options). Moved it to tests/universal/ accordingly — no test content changed.

Validation: pytest tests/unit before and after this change shows identical failure/error signatures (105 failed, 42 errors), confirming no functional changes — only reorganization, as required by the migration principles in #6048.