Ensure response stream is only closed after request stream is closed #595

Problem

When handling an invocation over HTTP, the SDK must not close the HTTP response stream before the request stream has been fully consumed/closed. If the response stream is closed while the request stream is still open, the runtime may interpret this as a connection abort and log warnings or trigger unnecessary retries.

This issue was discovered and fixed in the TypeScript SDK: restatedev/sdk-typescript#651

Expected behavior

After the SDK has flushed all output to the response stream, it should drain/await the request stream EOS (end-of-stream) before closing the response stream. This ensures a clean HTTP connection lifecycle:

1. SDK finishes writing all response data
2. SDK waits for the request stream to be fully closed (read until EOF or done)
3. SDK closes the response stream

Action items

• Review the HTTP handler code to check if the response stream can be closed before the request stream is fully consumed
• If susceptible, implement a fix to drain the request stream before closing the response stream
• Verify with testing