test(sandbox): mam_sandbox also drops HERDR_SESSION (reviews 63919c5d / 191f0ec3, non-blocking)

The caller-server helpers read it, so tests branched on whether they ran in
an agent pane. L2 covers it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff

tests/test_herdr_real_server_leak.py

@@ -72,7 +72,8 @@ def test_l1_a_directory_named_shim_is_still_skipped(mam_sandbox, mock_herdr):
 
 PANE_VARS = {"HERDR_SOCKET_PATH": "/nonexistent/sessions/real/herdr.sock",
              "HERDR_PANE_ID": "w9:p1", "HERDR_TAB_ID": "w9:t1",
-             "HERDR_WORKSPACE_ID": "w9", "HERDR_SESSION_NAME": "real"}
+             "HERDR_WORKSPACE_ID": "w9", "HERDR_SESSION_NAME": "real",
+             "HERDR_SESSION": "real"}
 
 
 @pytest.fixture