[Bug]: ValueError with RetrieveUserProxyAgent's default client

[lazToum]

Describe the bug

In RetrieveUserProxyAgent, when using a VectorDB instance for the vector_db argument, the client should be the one that the VectorDB instance uses.
https://github.com/ag2ai/ag2/blob/main/autogen/agentchat/contrib/retrieve_user_proxy_agent.py#L264

Steps to reproduce

• Create a new chromadb client:

  import chromadb
  from chromadb.config import Settings
  client = chromadb.PersistentClient(path="/path/to/documents", settings=Settings(anonymized_telemetry=False)
  

• Try to use this client in a RetrieveUserProxyAgent:

  from autogen.agentchat.contrib.retrieve_user_proxy_agent import RetrieveUserProxyAgent
  from autogen.agentchat.contrib.vectordb.chromadb import ChromaVectorDB
  from chromadb.utils.embedding_functions import SentenceTransformerEmbeddingFunction
  agent = RetrieveUserProxyAgent(
      ...,
      retrieve_config={
          ...,
          "vector_db": ChromaVectorDB(
              client=client,
              embedding_function=SentenceTransformerEmbeddingFunction(model_name="all-MiniLM-L6-v2")
          ),
         ...
   )
  

• Trying to use this agent gives:
  ValueError: An instance of Chroma already exists for ephemeral with different settings:

  ...site-packages/autogen/agentchat/contrib/retrieve_user_proxy_agent.py#line=265), in RetrieveUserProxyAgent.__init__(self, name, human_input_mode, is_termination_msg, retrieve_config, **kwargs)
      264 self._client = self._retrieve_config.get("client", None)
      265 if self._client is None:
  --> 266     self._client = chromadb.Client()
  ....
  

Model Used

No response

Expected Behavior

Not to throw an error and use the provided client.

Screenshots and logs

No response

Additional Information

Maybe related: microsoft/autogen#2829