fix: add aiohttp to gateway Dockerfile for Slack async client (#6719)

## Summary

- Add `aiohttp` to `Dockerfile.gateway` pip install list — required by
`slack-sdk`'s `AsyncWebClient`

## Problem

The Slack router's `_stream_slack_response` imports `AsyncWebClient` in
a background task (after the HTTP 200 ACK). `AsyncWebClient` depends on
`aiohttp`, which wasn't installed in the Docker image. This caused:

1. `ModuleNotFoundError: No module named 'aiohttp'` in the background
task
2. The exception propagates through Starlette's ASGI response cycle,
corrupting the HTTP keep-alive connection
3. The next test reusing that connection sees `Server disconnected
without sending a response`
4. **Non-deterministic failures** — whichever test lands on the
corrupted connection fails

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] New and existing tests pass locally with my changes

Co-authored-by: Yash Pratap Solanky <101447028+ysolanky@users.noreply.github.com>

Author: Mustafa-Esoofally

libs/agno/tests/system/Dockerfile.gateway

@@ -37,7 +37,8 @@ RUN pip install --no-cache-dir \
     a2a-sdk \
     agui \
     ag-ui-protocol \
-    slack-sdk
+    slack-sdk \
+    aiohttp
 
 # Copy the gateway server script
 WORKDIR /app