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

tests/test_agent_image_support.py

@@ -13,7 +13,7 @@ from app.agent.tools.impl.send_local_file import SendLocalFileInput
 from app.agent import MoviePilotAgent, AgentChain
 from app.chain.message import MessageChain
 from app.core.config import settings
-from app.helper.llm import LLMHelper
+from app.agent.llm import LLMHelper
 from app.modules.discord import DiscordModule
 from app.modules.qqbot import QQBotModule
 from app.modules.slack import SlackModule