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.

agent/model_factory.py

@@ -0,0 +1,68 @@
+"""
+Model factory for creating provider-specific chat models.
+
+Supports multiple AI providers with native integrations where available:
+- DeepSeek: Uses ChatDeepSeek for native tool calling support
+- OpenAI: Uses ChatOpenAI
+- Others: Fall back to ChatOpenAI (OpenAI-compatible endpoints)
+"""
+
+from typing import Any
+from langchain_openai import ChatOpenAI
+from langchain_deepseek import ChatDeepSeek
+
+
+def create_model(
+    basemodel: str,
+    api_key: str,
+    base_url: str,
+    temperature: float,
+    timeout: int
+) -> Any:
+    """
+    Create appropriate chat model based on provider.
+
+    Args:
+        basemodel: Model identifier (e.g., "deepseek/deepseek-chat", "openai/gpt-4")
+        api_key: API key for the provider
+        base_url: Base URL for API endpoint
+        temperature: Sampling temperature (0-1)
+        timeout: Request timeout in seconds
+
+    Returns:
+        Provider-specific chat model instance
+
+    Examples:
+        >>> model = create_model("deepseek/deepseek-chat", "key", "url", 0.7, 30)
+        >>> isinstance(model, ChatDeepSeek)
+        True
+
+        >>> model = create_model("openai/gpt-4", "key", "url", 0.7, 30)
+        >>> isinstance(model, ChatOpenAI)
+        True
+    """
+    # Extract provider from basemodel (format: "provider/model-name")
+    provider = basemodel.split("/")[0].lower() if "/" in basemodel else "unknown"
+
+    if provider == "deepseek":
+        # Use native ChatDeepSeek for DeepSeek models
+        # Extract model name without provider prefix
+        model_name = basemodel.split("/", 1)[1] if "/" in basemodel else basemodel
+
+        return ChatDeepSeek(
+            model=model_name,
+            api_key=api_key,
+            base_url=base_url,
+            temperature=temperature,
+            timeout=timeout
+        )
+    else:
+        # Use ChatOpenAI for OpenAI and OpenAI-compatible endpoints
+        # (Anthropic, Google, Qwen, etc. via compatibility layer)
+        return ChatOpenAI(
+            model=basemodel,
+            api_key=api_key,
+            base_url=base_url,
+            temperature=temperature,
+            timeout=timeout
+        )