feat(auth): Complete OIDC security refactor and modernize test suite

- Refactored OIDC flow to implement PKCE, state/nonce validation, and BFF pattern.
- Centralized configuration in Settings class (DEV_MODE, FRONTEND_URL, OIDC_REDIRECT_URI).
- Updated auth routers to use conditional secure cookie flags based on DEV_MODE.
- Modernized and cleaned up test suite by removing legacy Streamlit tests.
- Fixed linting errors and unused imports across the backend.

backend/src/ea_chatbot/graph/nodes/clarification.py

@@ -1,4 +1,3 @@
-from langchain_core.messages import AIMessage
 from ea_chatbot.graph.state import AgentState
 from ea_chatbot.config import Settings
 from ea_chatbot.utils.llm_factory import get_llm_model

backend/src/ea_chatbot/graph/nodes/coder.py

@@ -1,7 +1,7 @@
 from ea_chatbot.graph.state import AgentState
 from ea_chatbot.config import Settings
 from ea_chatbot.utils.llm_factory import get_llm_model
-from ea_chatbot.utils import helpers, database_inspection
+from ea_chatbot.utils import database_inspection
 from ea_chatbot.utils.logging import get_logger, LangChainLoggingHandler
 from ea_chatbot.graph.prompts.coder import CODE_GENERATOR_PROMPT
 from ea_chatbot.schemas import CodeGenerationResponse

backend/src/ea_chatbot/graph/nodes/executor.py

@@ -2,7 +2,7 @@ import io
 import sys
 import traceback
 from contextlib import redirect_stdout
-from typing import Any, Dict, List, TYPE_CHECKING
+from typing import TYPE_CHECKING
 import pandas as pd
 from matplotlib.figure import Figure
 

backend/src/ea_chatbot/graph/nodes/researcher.py

@@ -1,4 +1,3 @@
-from langchain_core.messages import AIMessage
 from langchain_openai import ChatOpenAI
 from langchain_google_genai import ChatGoogleGenerativeAI
 from ea_chatbot.graph.state import AgentState

backend/src/ea_chatbot/graph/nodes/summarizer.py

@@ -1,4 +1,3 @@
-from langchain_core.messages import AIMessage
 from ea_chatbot.graph.state import AgentState
 from ea_chatbot.config import Settings
 from ea_chatbot.utils.llm_factory import get_llm_model

backend/src/ea_chatbot/graph/state.py

@@ -1,4 +1,4 @@
-from typing import TypedDict, Annotated, List, Dict, Any, Optional
+from typing import Annotated, List, Dict, Any, Optional
 from langchain_core.messages import BaseMessage
 from langchain.agents import AgentState as AS
 import operator