tpu-client-next: wait_idle instead of close for endpoint

[KirillLykov]

Problem

Here we close endpoint. But I think it makes sense to use endpoint.wait_idle().await; to give the server a fair chance to finalize all the connections. Not sure, requires testing.

Proposed Solution

[shibu0x]

I checked the test cases but couldn’t find any that cover this behavior.

Adding .wait_idle() does make the shutdown smoother and allows the connection to close gracefully, although it may take slightly longer than .close() around 50-500 ms depending on the workload.

Overall, it seems like a good addition for a more graceful teardown.

We can also keep both .close() and .wait_idle(), so it first initiates the close on active connections and then waits for the endpoint to finish and clean up everything else.