feat: add agenthub_config kwarg to factory functions

[radu-mocanu]

Summary

• new agenthub_config: str | None = None kwarg on get_chat_model and get_embedding_model
• when set, overrides client_settings.agenthub_config for that call via model_copy (caller's settings instance is not mutated)
• enables low-code agent runtimes to pass per-execution AgentHub config (agentsruntime, agentsplayground, agentsevals, …) without rebuilding settings

Why

agentsruntime header value should not be hardcoded. each caller should be able to provide their custom requesting-product, while the default has to be None to properly fix the breaking change that was introduced for coded agents UiPath/uipath-langchain-python#825. the default value will be drop as part of the next release (as soon as low code agents are passing explicit values), to avoid breaking changes

prep work for fixing UiPath/uipath-langchain-python#825. the chat_model_factory in uipath-langchain-python silently drops its agenthub_config arg on the new-clients path because the upstream factory had nowhere to forward it. this adds the missing parameter so the next PR can plumb it through.