RetrieveUserProxyAgent, use context_max_tokens from retrieve_config if provided

[r-b-g-b]

Why are these changes needed?

ragproxyagent = RetrieveUserProxyAgent(
    name="ragproxyagent",
    human_input_mode="NEVER",
    max_consecutive_auto_reply=3,
    retrieve_config={
        "task": "qa",
        "docs_path": ["mypath.md"],
        "chunk_token_size": 500,
        "context_max_tokens": 2000,
        "client": docdb,
        "embedding_model": "all-mpnet-base-v2",
        "get_or_create": True,
    },
    code_execution_config=False,  # set to False if you don't want to execute the code
)

RetrieveUserProxyAgent ignores the retrieve_config["context_max_tokens"] and always uses the default of self._max_tokens * 0.8. This is in conflict with the documented use:

- context_max_tokens (Optional, int): the context max token size for the retrieve chat.
    If key not provided, a default size `max_tokens * 0.8` will be used.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[ekzhu]

Thanks for the PR! Cc @thinkall

[thinkall]

Good catch! Thank you @r-b-g-b .