Rafe: Update LLMService type hints (#9131)

### What problem does this PR solve?

- Add Generator return type annotation for tts method
- Import typing.Generator for type hints

### Type of change

- [x] Refactoring

api/db/services/llm_service.py

 import logging
 import re
 from functools import partial
+from typing import Generator
 
 from langfuse import Langfuse
 
                 mdlnm += "___OpenAI-API"
             elif fid == "VLLM":
                 mdlnm += "___VLLM"
-                
             objs = cls.query(tenant_id=tenant_id, llm_name=mdlnm, llm_factory=fid)
         if not objs:
             return
 
         return txt
 
-    def tts(self, text: str) -> None:
+    def tts(self, text: str) -> Generator[bytes, None, None]:
         if self.langfuse:
             span = self.trace.span(name="tts", input={"text": text})