Skip downstream benchmark tests when benchmarks package unavailable

These tests require the benchmarks/ package on sys.path which is only
available when running from the repo root. They now skip gracefully
in CI and pip-installed environments.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: TimoLassmann

tests/python/test_downstream_foundation.py

@@ -11,6 +11,8 @@
 
 import pytest
 
+pytest.importorskip("benchmarks", reason="benchmarks package not on sys.path (run from repo root)")
+
 
 # ======================================================================
 # provenance.py

tests/python/test_downstream_integration.py

@@ -14,6 +14,8 @@
 
 import pytest
 
+pytest.importorskip("benchmarks", reason="benchmarks package not on sys.path (run from repo root)")
+
 
 # ======================================================================
 # Module import tests