(testing) add unit tests for LLMCachingHandler Class (#6279)

* add unit testing for test_async_set_cache

* test test_async_log_cache_hit_on_callbacks

* assert the correct response type is returned

* test_convert_cached_result_to_model_response

* unit testing for caching handler

litellm/caching/caching_handler.py

@@ -512,7 +512,16 @@ class LLMCachingHandler:
         model: str,
         args: Tuple[Any, ...],
         custom_llm_provider: Optional[str] = None,
-    ) -> Optional[Any]:
+    ) -> Optional[
+        Union[
+            ModelResponse,
+            TextCompletionResponse,
+            EmbeddingResponse,
+            RerankResponse,
+            TranscriptionResponse,
+            CustomStreamWrapper,
+        ]
+    ]:
         """
         Internal method to process the cached result