[router] add py binding unit tests to coverage 80% (#10043)

sgl-router/py_test/conftest.py

@@ -0,0 +1,8 @@
+import sys
+from pathlib import Path
+
+# Ensure local sources in py_src are importable ahead of any installed package
+_ROOT = Path(__file__).resolve().parents[1]
+_SRC = _ROOT / "py_src"
+if str(_SRC) not in sys.path:
+    sys.path.insert(0, str(_SRC))