test(api): Add extended agent stream tests and fix type annotations

backend/src/ea_chatbot/api/routers/agent.py

@@ -14,6 +14,8 @@ import base64
 from langchain_core.runnables.config import RunnableConfig
 from langchain_core.messages import BaseMessage
 
+from ea_chatbot.graph.state import AgentState
+
 router = APIRouter(prefix="/chat", tags=["agent"])
 
 async def stream_agent_events(
@@ -27,7 +29,7 @@ async def stream_agent_events(
     Generator that invokes the LangGraph agent and yields SSE formatted events.
     Persists assistant responses and plots to the database.
     """
-    initial_state = {
+    initial_state: AgentState = {
         "messages": messages, 
         "question": message,
         "summary": summary,
@@ -38,7 +40,8 @@ async def stream_agent_events(
         "code_output": None,
         "error": None,
         "plots": [],
-        "dfs": {}
+        "dfs": {},
+        "iterations": 0
     }
     
     config: RunnableConfig = {"configurable": {"thread_id": thread_id}}