fix: Add client attribute type annotation to BaseOpenAIClient

Fixes pyright type checking error by declaring the client attribute
in the base class. All concrete implementations (OpenAIClient,
AzureOpenAILLMClient, OpenAIGenericClient) initialize this attribute,
and it needs to be accessible for type checking in OpenAIRerankerClient.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

graphiti_core/llm_client/openai_base_client.py

@@ -21,6 +21,7 @@ from abc import abstractmethod
 from typing import Any, ClassVar
 
 import openai
+from openai import AsyncAzureOpenAI, AsyncOpenAI
 from openai.types.chat import ChatCompletionMessageParam
 from pydantic import BaseModel
 
@@ -48,6 +49,9 @@ class BaseOpenAIClient(LLMClient):
     # Class-level constants
     MAX_RETRIES: ClassVar[int] = 2
 
+    # Instance attribute (initialized in subclasses)
+    client: AsyncOpenAI | AsyncAzureOpenAI
+
     def __init__(
         self,
         config: LLMConfig | None = None,