Implement MS-TSGU client channel close handling

`GwClient` does not send an MS-TSGU close message when a client connection ends, and `poll_shutdown` is a no-op.

Implement client-initiated channel closure so consumers such as `ironrdp-agent gw-forward` can propagate a TCP half-close and complete an orderly RD Gateway teardown.

> [!NOTE]
> Human-tuned, LLM-assisted content.

Locate GwClient and its no-op poll_shutdown implementation, then trace how ironrdp-agent gw-forward handles connection shutdown. Confirm that client-initiated closure sends the MS-TSGU close message, propagates a TCP half-close, and completes an orderly RD Gateway teardown.

Written by the indexing model from the issue text.