feat: add LangChain-compatible mock chat model wrapper

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/unit/test_mock_provider.py

@@ -1,5 +1,7 @@
 import pytest
+import asyncio
 from agent.mock_provider.mock_ai_provider import MockAIProvider
+from agent.mock_provider.mock_langchain_model import MockChatModel
 
 
 def test_mock_provider_rotates_stocks():
@@ -32,3 +34,41 @@ def test_mock_provider_valid_json_tool_calls():
     provider = MockAIProvider()
     response = provider.generate_response("2025-01-01", step=0)
     assert "[calls tool_get_price" in response or "get_price" in response.lower()
+
+
+def test_mock_chat_model_invoke():
+    """Test synchronous invoke returns proper message format"""
+    model = MockChatModel(date="2025-01-01")
+
+    messages = [{"role": "user", "content": "Analyze the market"}]
+    response = model.invoke(messages)
+
+    assert hasattr(response, "content")
+    assert "AAPL" in response.content
+    assert "<FINISH_SIGNAL>" in response.content
+
+
+def test_mock_chat_model_ainvoke():
+    """Test asynchronous invoke returns proper message format"""
+    async def run_test():
+        model = MockChatModel(date="2025-01-02")
+        messages = [{"role": "user", "content": "Analyze the market"}]
+        response = await model.ainvoke(messages)
+
+        assert hasattr(response, "content")
+        assert "MSFT" in response.content
+        assert "<FINISH_SIGNAL>" in response.content
+
+    asyncio.run(run_test())
+
+
+def test_mock_chat_model_different_dates():
+    """Test that different dates produce different responses"""
+    model1 = MockChatModel(date="2025-01-01")
+    model2 = MockChatModel(date="2025-01-02")
+
+    msg = [{"role": "user", "content": "Trade"}]
+    response1 = model1.invoke(msg)
+    response2 = model2.invoke(msg)
+
+    assert response1.content != response2.content