feat: add LangChain-compatible mock chat model wrapper

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

agent/mock_provider/__init__.py

@@ -1,4 +1,5 @@
 """Mock AI provider for development mode testing"""
 from .mock_ai_provider import MockAIProvider
+from .mock_langchain_model import MockChatModel
 
-__all__ = ["MockAIProvider"]
+__all__ = ["MockAIProvider", "MockChatModel"]