feat: add model factory for provider-specific chat models

Implements factory pattern to create appropriate chat model based on
provider prefix in basemodel string.

Supported providers:
- deepseek/*: Uses ChatDeepSeek (native tool calling)
- openai/*: Uses ChatOpenAI
- others: Fall back to ChatOpenAI (OpenAI-compatible)

This enables native DeepSeek integration while maintaining backward
compatibility with existing OpenAI-compatible providers.

tests/unit/test_model_factory.py

@@ -0,0 +1,108 @@
+"""Unit tests for model factory - provider-specific model creation"""
+
+import pytest
+from unittest.mock import Mock, patch
+from agent.model_factory import create_model
+
+
+class TestModelFactory:
+    """Tests for create_model factory function"""
+
+    @patch('agent.model_factory.ChatDeepSeek')
+    def test_create_model_deepseek(self, mock_deepseek_class):
+        """Test that DeepSeek models use ChatDeepSeek"""
+        mock_model = Mock()
+        mock_deepseek_class.return_value = mock_model
+
+        result = create_model(
+            basemodel="deepseek/deepseek-chat",
+            api_key="test-key",
+            base_url="https://api.deepseek.com",
+            temperature=0.7,
+            timeout=30
+        )
+
+        # Verify ChatDeepSeek was called with correct params
+        mock_deepseek_class.assert_called_once_with(
+            model="deepseek-chat",  # Extracted from "deepseek/deepseek-chat"
+            api_key="test-key",
+            base_url="https://api.deepseek.com",
+            temperature=0.7,
+            timeout=30
+        )
+        assert result == mock_model
+
+    @patch('agent.model_factory.ChatOpenAI')
+    def test_create_model_openai(self, mock_openai_class):
+        """Test that OpenAI models use ChatOpenAI"""
+        mock_model = Mock()
+        mock_openai_class.return_value = mock_model
+
+        result = create_model(
+            basemodel="openai/gpt-4",
+            api_key="test-key",
+            base_url="https://api.openai.com/v1",
+            temperature=0.7,
+            timeout=30
+        )
+
+        # Verify ChatOpenAI was called with correct params
+        mock_openai_class.assert_called_once_with(
+            model="openai/gpt-4",
+            api_key="test-key",
+            base_url="https://api.openai.com/v1",
+            temperature=0.7,
+            timeout=30
+        )
+        assert result == mock_model
+
+    @patch('agent.model_factory.ChatOpenAI')
+    def test_create_model_anthropic(self, mock_openai_class):
+        """Test that Anthropic models use ChatOpenAI (via compatibility)"""
+        mock_model = Mock()
+        mock_openai_class.return_value = mock_model
+
+        result = create_model(
+            basemodel="anthropic/claude-sonnet-4.5",
+            api_key="test-key",
+            base_url="https://api.anthropic.com/v1",
+            temperature=0.7,
+            timeout=30
+        )
+
+        # Verify ChatOpenAI was used (Anthropic via OpenAI-compatible endpoint)
+        mock_openai_class.assert_called_once()
+        assert result == mock_model
+
+    @patch('agent.model_factory.ChatOpenAI')
+    def test_create_model_generic_provider(self, mock_openai_class):
+        """Test that unknown providers default to ChatOpenAI"""
+        mock_model = Mock()
+        mock_openai_class.return_value = mock_model
+
+        result = create_model(
+            basemodel="custom/custom-model",
+            api_key="test-key",
+            base_url="https://api.custom.com",
+            temperature=0.7,
+            timeout=30
+        )
+
+        # Should fall back to ChatOpenAI for unknown providers
+        mock_openai_class.assert_called_once()
+        assert result == mock_model
+
+    def test_create_model_deepseek_extracts_model_name(self):
+        """Test that DeepSeek model name is extracted correctly"""
+        with patch('agent.model_factory.ChatDeepSeek') as mock_class:
+            create_model(
+                basemodel="deepseek/deepseek-chat-v3.1",
+                api_key="key",
+                base_url="url",
+                temperature=0,
+                timeout=30
+            )
+
+            # Check that model param is just "deepseek-chat-v3.1"
+            call_kwargs = mock_class.call_args[1]
+            assert call_kwargs['model'] == "deepseek-chat-v3.1"