`_prepare_completion_params` uses `is_azure_openai_endpoint` to decide
whether to include the `model` parameter in requests — Azure OpenAI
endpoints embed the deployment name in the URL and reject a `model`
field. When the endpoint was resolved lazily from env vars, the flag
stayed at its pre-resolve `False` value, causing every lazily-inited
Azure OpenAI request to include `model` and fail.
Factor the classification into `_is_azure_openai_endpoint` and call
it from both `_normalize_azure_fields` and `_make_client_kwargs`.
Extend the lazy-build regression test to assert the flag flips to
`True` once the endpoint is resolved.
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