[ci] add aiohttp dep to chat-adapter-slack

slack_sdk.web.async_client.AsyncWebClient imports aiohttp at runtime but
it wasn't in chat-adapter-slack's dependency list, so every import of the
adapter raised ModuleNotFoundError. This hid behind import-time lazy
loading until CI reached the test step (see bf64041 and e3c5f94 for how
that became visible). Adding aiohttp>=3.9 to match slack-sdk's extra
and flips all 24 previously-failing slack adapter tests to passing.

Full suite now: 2094 passed, 9 skipped (gated integration tests).

Author: Worker Agent

packages/chat-adapter-slack/pyproject.toml

@@ -23,6 +23,7 @@ dependencies = [
     "slack-sdk>=3.26",
     "httpx>=0.27",
     "cryptography>=43.0",
+    "aiohttp>=3.9",
 ]
 
 [project.urls]