test(streamable-http): avoid assert-only coverage gap on py311

Author: mrutunjay-kinagi

tests/shared/test_streamable_http.py

@@ -1848,7 +1848,8 @@ async def exercise_client() -> None:
 
     anyio.run(exercise_client)
 
-    assert start_count == 1
+    if start_count != 1:
+        raise AssertionError(f"Expected exactly one GET stream start, got {start_count}")
 
 
 @pytest.mark.anyio