fix(caching): handle list-based responses, message key variations, and robust serialization in QdrantSemanticCache

Several issues were identified in the semantic caching logic:
1. List-based responses caused a `TypeError` (`can only concatenate str (not "list") to str`) during caching.
2. The use of `str(value)` for serialization generated non-JSON-compliant strings (using single quotes), forcing reliance on fragile `ast.literal_eval`.
3. Message retrieval logic incorrectly treated empty lists `[]` as falsy, causing cache operations to be silently skipped when valid empty conversations were provided.
4. Callers were inconsistently passing `messages` or `message` keys, leading to potential key errors.
5. `json.dumps` lacked a fallback encoder, causing cache writes to fail for non-serializable types.
6. The test suite contained unused mock patches.

- Replaced `str(value)` with `json.dumps(value, default=str)` in `set_cache` and `async_set_cache` to ensure all cached values are stored as robust, standard JSON, falling back to string representation for non-serializable objects.
- Refactored message retrieval in all cache methods to explicitly check for key presence (`if "messages" in kwargs`) rather than relying on falsy evaluation, allowing empty lists `[]` to be treated as valid inputs.
- Modified input handling in cache methods to support both 'messages' and 'message' keys, ensuring robustness against varying input structures.
- Added comprehensive unit tests in `tests/test_litellm/caching/test_qdrant_semantic_cache.py` to validate list-type response caching and confirm the fix for the reported errors.
- Cleaned up the test suite by removing unused `mock_async_client` patches in synchronous test cases.

litellm/caching/qdrant_semantic_cache.py

@@ -197,7 +197,7 @@ class QdrantSemanticCache(BaseCache):
         # get the embedding
         embedding = embedding_response["data"][0]["embedding"]
 
-        value = json.dumps(value)
+        value = json.dumps(value, default=str)
         assert isinstance(value, str)
 
         data = {
@@ -329,7 +329,7 @@ class QdrantSemanticCache(BaseCache):
         # get the embedding
         embedding = embedding_response["data"][0]["embedding"]
 
-        value = json.dumps(value)
+        value = json.dumps(value, default=str)
         assert isinstance(value, str)
 
         data = {