Fix incorrect import path for AzureAIChatCompletionsModel (#30417)
Fixes #30416
Correct the import path for `AzureAIChatCompletionsModel` in the
`_init_chat_model_helper` function.
* Update the import statement in
`libs/langchain/langchain/chat_models/base.py` to `from
langchain_azure_ai.chat_models import AzureAIChatCompletionsModel`.
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