fix(ag-ui): eliminate unnecessary mcp import

Eliminate the unnecessary import of mcp in ag_ui.py, which was causing
issues when running the tests in CI as mcp is not always available.

Author: stevenh

pydantic_ai_slim/pydantic_ai/ag_ui.py

@@ -12,7 +12,18 @@
 from collections.abc import Iterable, Mapping, Sequence
 from dataclasses import dataclass, field
 from enum import Enum
-from typing import TYPE_CHECKING, Any, Callable, Final, Generic, Protocol, TypeVar, cast, runtime_checkable
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Callable,
+    Final,
+    Generic,
+    NoReturn,
+    Protocol,
+    TypeVar,
+    cast,
+    runtime_checkable,
+)
 
 try:
     from ag_ui.core import (
@@ -67,7 +78,6 @@
 from ._output import OutputType
 from ._parts_manager import ModelResponsePartsManager
 from .agent import RunOutputDataT
-from .mcp import ToolResult
 from .messages import (
     AgentStreamEvent,
     FinalResultEvent,
@@ -644,7 +654,7 @@ def _tool_call(self, tool: ToolAGUI) -> Tool[AgentDepsT]:
             A PydanticAI `Tool` object that calls the AG-UI tool.
         """
 
-        def _tool_stub(*args: Any, **kwargs: Any) -> ToolResult:
+        def _tool_stub(*args: Any, **kwargs: Any) -> NoReturn:
             """Stub function which is never called.
 
             Returns:
@@ -655,7 +665,6 @@ def _tool_stub(*args: Any, **kwargs: Any) -> ToolResult:
             """
             raise _UnexpectedToolCallError(tool_name=tool.name)  # pragma: no cover
 
-        # TODO(steve): See it we can avoid the cast here.
         return cast(
             'Tool[AgentDepsT]',
             Tool.from_schema(