Gracefully shutdown is hang while a SSE connection is established

[greenhat616]

Describe the bug
While using streamable-http transport. The SseBody donnot handle the signal of gracefully shutdown, thus, blocking the shutdown process until client disconnect.

To Reproduce

1. start the server

RUST_LOG=trace cargo run --example servers_counter_streamhtt

2. create a client connection to server via @modelcontextprotocol/inspector

3. press CTRL + C to trigger server exits

I noted the sse transport has similar issue, but I don't dig it deep.

Expected behavior
While trigger cancelled, the server will exits immediately.

Logs
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Additional context
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