chore: create union for working with message content (#939)

examples/servers/simple-streamablehttp/mcp_simple_streamablehttp/server.py

@@ -45,14 +45,7 @@ def main(
     app = Server("mcp-streamable-http-demo")
 
     @app.call_tool()
-    async def call_tool(
-        name: str, arguments: dict
-    ) -> list[
-        types.TextContent
-        | types.ImageContent
-        | types.AudioContent
-        | types.EmbeddedResource
-    ]:
+    async def call_tool(name: str, arguments: dict) -> list[types.Content]:
         ctx = app.request_context
         interval = arguments.get("interval", 1.0)
         count = arguments.get("count", 5)