fix(slackbot): use MCPServerStreamableHTTP for FastMCP.app (#1271)

FastMCP.app uses streamable HTTP transport (POST-based), not SSE (GET-based).
MCPServerHTTP was returning 405 Method Not Allowed.

Author: zzstoatzz

examples/slackbot/src/slackbot/core.py

@@ -12,7 +12,7 @@
 from prefect.logging.loggers import get_logger
 from prefect.variables import Variable
 from pydantic_ai import Agent, RunContext
-from pydantic_ai.mcp import MCPServerHTTP
+from pydantic_ai.mcp import MCPServerStreamableHTTP
 from pydantic_ai.messages import ModelMessage, ModelMessagesTypeAdapter
 from pydantic_ai.models import KnownModelName, Model
 from pydantic_ai.models.anthropic import AnthropicModel
@@ -47,7 +47,7 @@
 
 GITHUB_API_TOKEN = Secret.load(settings.github_token_secret_name, _sync=True).get()
 
-slack_search_mcp = MCPServerHTTP(
+slack_search_mcp = MCPServerStreamableHTTP(
     url="https://marvin-slack-thread-assets.fastmcp.app/mcp",
 )