fix(event-handler): fix UP045 lint error in oauth2.py validator signature

With `from __future__ import annotations` present, ruff UP045 requires `str | None` instead of `Optional[str]` in the validator signature.

- Remove Optional from typing imports
- Convert validator signature to str | None syntax

Part of #8088

Signed-off-by: hirenkumar-n-dholariya <hirenkumarnd@gmail.com>

Author: hirenkumar-n-dholariya

aws_lambda_powertools/event_handler/openapi/swagger_ui/oauth2.py

@@ -2,7 +2,7 @@
 from __future__ import annotations
 
 import warnings
-from typing import Optional, Sequence
+from typing import Sequence
 
 from pydantic import BaseModel, Field, field_validator
 
@@ -49,7 +49,7 @@ class OAuth2Config(BaseModel):
     model_config = MODEL_CONFIG_ALLOW
 
     @field_validator("clientSecret")
-    def client_secret_only_on_dev(cls, v: Optional[str]) -> Optional[str]:
+    def client_secret_only_on_dev(cls, v: str | None) -> str | None:
         if not v:
             return None