fix: preserve pythonpath for built-in mcp servers (#5117)

tests/test_builtin_mcp_pythonpath.py

@@ -0,0 +1,22 @@
+import os
+
+from src.builtin_mcp import builtin_python_env
+
+
+def test_builtin_python_env_preserves_existing_pythonpath(monkeypatch):
+    monkeypatch.setenv(
+        "PYTHONPATH",
+        os.pathsep.join(["/app/venv/lib/python3.13/site-packages", "/app", "/extra"]),
+    )
+
+    env = builtin_python_env("/app")
+
+    assert env == {
+        "PYTHONPATH": os.pathsep.join(["/app", "/app/venv/lib/python3.13/site-packages", "/extra"])
+    }
+
+
+def test_builtin_python_env_uses_app_root_without_existing_pythonpath(monkeypatch):
+    monkeypatch.delenv("PYTHONPATH", raising=False)
+
+    assert builtin_python_env("/srv/odysseus") == {"PYTHONPATH": "/srv/odysseus"}