fix: Import Self from typing_extensions for Python 3.10 compat

mypy targets python_version = "3.10" but typing.Self was added in
3.11. Use unconditional typing_extensions import and remove the
unnecessary from __future__ import annotations.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jsonbailey

packages/sdk/server-ai/src/ldai/models.py

@@ -1,13 +1,8 @@
-from __future__ import annotations
-
 import warnings
 from dataclasses import dataclass, field
 from typing import Any, Callable, Dict, List, Literal, Optional, Union
 
-try:
-    from typing import Self
-except ImportError:
-    from typing_extensions import Self
+from typing_extensions import Self
 
 
 @dataclass