fix(slackbot): create MCP server per agent run to avoid cancel scope error (#1289)

The module-level MCPServerStreamableHTTP singleton was shared across
concurrent asyncio.create_task handlers. streamablehttp_client uses
anyio.create_task_group() internally, binding a cancel scope to the
entering task. When the last handler to exit was a different task than
the one that opened the connection, anyio raised "Attempted to exit
cancel scope in a different task than it was entered in".

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: zzstoatzz

examples/slackbot/src/slackbot/core.py

@@ -47,10 +47,6 @@
 
 GITHUB_API_TOKEN = Secret.load(settings.github_token_secret_name, _sync=True).get()
 
-slack_search_mcp = MCPServerStreamableHTTP(
-    url="https://marvin-slack-thread-assets.fastmcp.app/mcp",
-)
-
 logger = get_logger(__name__)
 
 
@@ -178,6 +174,9 @@ def create_agent(
             api_key=Secret.load(settings.anthropic_key_secret_name, _sync=True).get(),  # type: ignore
         ),
     )
+    slack_search_mcp = MCPServerStreamableHTTP(
+        url="https://marvin-slack-thread-assets.fastmcp.app/mcp",
+    )
     agent = Agent[
         UserContext, str
     ](