Preserve isolated Python tools in the DOCX lint test

Why: The test replaced PYTHONPATH with the house-style source path, hiding the fixed Python wheels installed inside the candidate sandbox.

Evidence: Isolated Backend TestLintErrorContinues replay passed with locked wheel acquisition and no host site-packages; SELF receipt /tmp/dependency-task3-backend-lint-probe-20261014-01a1345f.json.

Impact: Test environment composition only; production runtime behavior is unchanged.

Refs: Backend PR #196; Dev PR #537; Documents plan 5099e12

internal/runtime/llm_verify_test.go

@@ -100,6 +100,9 @@ func TestLintErrorContinues(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	if existing := os.Getenv("PYTHONPATH"); existing != "" {
+		abs += string(os.PathListSeparator) + existing
+	}
 	t.Setenv("PYTHONPATH", abs)
 	docx := badDocx(t)
 	turns := padTurns(