feat: Implement Dify LLM provider and update related configurations and tests

engine/providers/llm/__init__.py

@@ -1,5 +1,6 @@
 """LLM providers."""
 
+from providers.llm.dify import DifyLLMService
 from providers.llm.openai import MockLLMService, OpenAILLMService
 
 try:  # pragma: no cover - import depends on optional sibling SDK
@@ -8,6 +9,7 @@ except Exception:  # pragma: no cover - provider remains lazily available via fa
     FastGPTLLMService = None  # type: ignore[assignment]
 
 __all__ = [
+    "DifyLLMService",
     "FastGPTLLMService",
     "MockLLMService",
     "OpenAILLMService",