refactor: migrate LLM helper to agent module and add unified LLM API endpoints

- Move LLMHelper and related logic from app.helper.llm to app.agent.llm.helper
- Update all imports to reference new LLMHelper location
- Introduce app/agent/llm/__init__.py for internal LLM adapter exports
- Add llm.py API router with endpoints for model listing, provider auth, and test calls
- Remove legacy LLM endpoints from system.py
- Update requirements for langchain-anthropic and anthropic
- Refactor test_llm_helper_testcall.py for async LLMHelper usage and new import paths

requirements.in

@@ -79,10 +79,12 @@ httpx[socks]~=0.28.1
 langchain~=1.2.15
 langchain-core~=1.3.2
 langchain-community~=0.4.1
+langchain-anthropic~=1.1.0
 langchain-openai~=1.2.1
 langchain-google-genai~=4.2.2
 langchain-deepseek~=1.0.1
 langgraph~=1.1.9
+anthropic>=0.57,<1
 openai~=2.32.0
 google-genai~=1.73.1
 ddgs~=9.10.0