chore: import upstream snapshot with attribution

examples/stories/sse_polling/client.py

@@ -0,0 +1,32 @@
+"""Call a tool whose SSE stream the server closes mid-flight; the call still completes. HTTP-only — no SSE on stdio."""
+
+import anyio
+from mcp_types import TextContent
+
+from mcp.client import Client
+from stories._harness import Target, run_client
+
+
+async def main(target: Target, *, mode: str = "auto") -> None:
+    async with Client(target, mode=mode) as client:
+        messages: list[str | None] = []
+
+        async def on_progress(progress: float, total: float | None, message: str | None) -> None:
+            messages.append(message)
+
+        with anyio.fail_after(10):
+            result = await client.call_tool("long_operation", {}, progress_callback=on_progress)
+
+        # The result arrived — the client transport survived the server-initiated close,
+        # reconnected with Last-Event-ID, and received the replayed response.
+        assert not result.is_error, result
+        assert isinstance(result.content[0], TextContent)
+        assert result.content[0].text == "resumed"
+
+        # "after-close" was emitted while no SSE stream was open; receiving it proves the
+        # event store buffered it and the reconnect replayed it.
+        assert messages == ["before-close", "after-close"], messages
+
+
+if __name__ == "__main__":
+    run_client(main)