What this PR does / why we need it

Relocates cross-backend parameterized tests from sdk/python/tests/integration/* into dedicated sdk/python/tests/universal/* directories, aligning the test suite with the 4-tier taxonomy proposed in #6048.

No test logic or behavior changes are introduced.

universal/offline_store/

Move all 10 test files from integration/offline_store/.

These tests use the shared environment fixture and are marked with @pytest.mark.universal_offline_stores, confirming that they run across multiple backend implementations.

universal/online_store/

Move the following files from integration/online_store/:

• test_universal_e2e.py
• test_universal_online.py
• test_universal_online_types.py
• test_push_features_to_online_store.py

universal/materialization/

Move:

• test_universal_materialization.py

from integration/materialization/.

Outlier corrections

Move tests that do not belong in the universal layer:

• test_hybrid_online_store.py → unit/

  • Uses only unittest.mock
  • Does not interact with external services

• test_remote_online_store.py → integration/permissions/auth/

  • Auth-specific integration coverage
  • Not cross-backend/universal behavior

Reference updates

Update references impacted by the test relocation:

• Fix 4 stale paths in Makefile

  • Cassandra target
  • Milvus target
  • Qdrant target
  • Additional stale reference

• Correct filename:

  • test_validation.py → test_dqm_validation.py

• Update:

  • docs/how-to-guides/adding-or-reusing-tests.md
  • sdk/python/tests/README.md

Which issue(s) this PR fixes

Closes #6051

Does this PR introduce a user-facing change?

NONE

Checks

• I've made sure the tests are passing
• My commits are signed off (git commit -s)
• My PR title follows the Conventional Commits format

Testing Strategy

• Unit tests (make test-python-unit)

  • 1907 passed
  • 35 skipped
  • 6 pre-existing PySpark/CUDA failures unrelated to this PR

• Integration tests

• Manual tests

• Testing is not required for this change

Additional Notes

This is my first contribution to Feast, so please let me know if there's anything I've overlooked or if you'd prefer a different approach.