fix: scope make test to core tests

[mturac]

Summary

• limit the Makefile test target to the repository core tests directory
• avoid collecting mcp_server/tests from the root pytest run
• keep the existing non-integration marker and backend disable flags unchanged

Closes #1180

Test plan

• make -n test
• DISABLE_FALKORDB=1 DISABLE_KUZU=1 DISABLE_NEPTUNE=1 uv run pytest tests/ -m "not integration" --collect-only -q

The collect-only run no longer reaches the reported mcp_server/tests import error. It stops later in this local environment on optional dependency setup for sentence_transformers/torch and voyageai.