test: cover external-exit-stack and session-stack teardown branches

Lines 151, 164, 170-171 were uncovered after stripping stale pragmas:
- 151 (__aenter__ with external exit stack)
- 164-171 (__aexit__ with external exit stack + session stack teardown)

Add two anyio tests: one for the external-exit-stack case verifying
the group doesn't touch the caller's stack, and one that populates
_session_exit_stacks to exercise the sequential aclose loop.

Author: VRTXOmega

tests/client/test_session_group.py

@@ -523,8 +523,37 @@ async def test_unreachable_streamable_http_error_is_catchable() -> None:
         "Expected to catch a connection error against an unreachable "
         "streamable-http server, but no exception was raised."
     )
-    assert not _is_cancel_scope_runtime_error(caught), (
-        "Regression of #915: connection error against an unreachable "
-        "streamable-http server was masked by an anyio cancel-scope "
-        f"RuntimeError. Got: {type(caught).__name__}: {caught}"
-    )
+    assert not _is_cancel_scope_runtime_error(caught), (
+        "Regression of #915: connection error against an unreachable "
+        "streamable-http server was masked by an anyio cancel-scope "
+        f"RuntimeError. Got: {type(caught).__name__}: {caught}"
+    )
+
+
+@pytest.mark.anyio
+async def test_session_group_with_external_exit_stack(
+    mock_exit_stack: mock.MagicMock,
+) -> None:
+    """__aenter__/__aexit__ skip stack management when given an external exit stack."""
+    group = ClientSessionGroup(exit_stack=mock_exit_stack)
+
+    async with group:
+        pass  # __aenter__ covered; __aexit__ triggers next
+
+    # mock_exit_stack's own __aenter__/aclose should NOT be called by
+    # ClientSessionGroup — it's the caller's responsibility.
+    mock_exit_stack.__aenter__.assert_not_called()
+    mock_exit_stack.aclose.assert_not_called()
+
+
+@pytest.mark.anyio
+async def test_session_group_teardown_closes_session_stacks() -> None:
+    """__aexit__ closes every session-level exit stack sequentially."""
+    session = mock.MagicMock(spec=mcp.ClientSession)
+    session_stack = mock.MagicMock(spec=contextlib.AsyncExitStack)
+
+    async with ClientSessionGroup() as group:
+        group._session_exit_stacks[session] = session_stack
+
+    session_stack.aclose.assert_awaited_once()
+