fix(core): add 'ag_ui' to optional dependency error check

Add "ag_ui" to the error message checking logic in __getattr__ to properly
handle import errors when ag-ui-protocol is not installed. This ensures users
get helpful error messages when trying to use AGUIProtocolHandler or other
AG-UI related components without the server optional dependencies installed.

Addresses feedback from PR #23 review comment.

Co-authored-by: OhYee <[email protected]>

Author: Copilot

agentrun/__init__.py

@@ -303,7 +303,7 @@ def __getattr__(name: str):
             return getattr(server, name)
         except ImportError as e:
             # 检查是否是缺少可选依赖导致的错误
-            if "fastapi" in str(e) or "uvicorn" in str(e):
+            if "fastapi" in str(e) or "uvicorn" in str(e) or "ag_ui" in str(e):
                 raise ImportError(
                     f"'{name}' requires the 'server' optional dependencies. "
                     "Install with: pip install agentrun-sdk[server]\n"