Fix secret_from_env import error for langchain v0.2

- Remove deprecated secret_from_env import from langchain_core.utils.utils
- Simplify ChatOpenRouter class to use direct os.environ.get()
- Remove unused Pydantic Field and SecretStr imports
- Fix ImportError with langchain-core 0.2.x compatibility

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

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

app.py

@@ -17,26 +17,21 @@ from langchain.text_splitter import RecursiveCharacterTextSplitter
 from langchain.schema import Document
 from langchain_openai import ChatOpenAI
 from langchain_core.prompts import PromptTemplate
-from langchain_core.utils.utils import secret_from_env
 from langchain.chains import LLMChain
 
 # Load environment variables
 load_dotenv()
 
 class ChatOpenRouter(ChatOpenAI):
-    openai_api_key: Optional[SecretStr] = Field(
-        alias="api_key", default_factory=lambda: secret_from_env("OPENROUTER_API_KEY", default=None)
-    )
-    
-    @property
-    def lc_secrets(self) -> dict[str, str]:
-        return {"openai_api_key": "OPENROUTER_API_KEY"}
-
     def __init__(self,
                  openai_api_key: Optional[str] = None,
                  **kwargs):
         openai_api_key = openai_api_key or os.environ.get("OPENROUTER_API_KEY")
-        super().__init__(base_url="https://openrouter.ai/api/v1", openai_api_key=openai_api_key, **kwargs)
+        super().__init__(
+            base_url="https://openrouter.ai/api/v1", 
+            openai_api_key=openai_api_key, 
+            **kwargs
+        )
 
 # Initialize OpenRouter model
 openrouter_model = ChatOpenRouter(