Feat: add xAI provider (#8781)

### What problem does this PR solve?

Add xAI provider (experimental feature, requires user feedback).

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

rag/llm/chat_model.py

@@ -568,6 +568,16 @@ class BaiChuanChat(Base):
         yield total_tokens
 
 
+class xAIChat(Base):
+    _FACTORY_NAME = "xAI"
+
+    def __init__(self, key, model_name="grok-3", base_url=None, **kwargs):
+        if not base_url:
+            base_url = "https://api.x.ai/v1"
+        super().__init__(key, model_name, base_url=base_url, **kwargs)
+        return
+
+
 class QWenChat(Base):
     _FACTORY_NAME = "Tongyi-Qianwen"