feat(openai): add Azure OpenAI client support

[kime541200]

Description

Fixes #49. Related to #99.

Feature

Adds native Azure OpenAI client initialization to the existing OpenAI provider.
When callers provide Azure OpenAI client kwargs such as azure_endpoint,
api_version, azure_ad_token, azure_ad_token_provider, or
azure_deployment, OpenAILanguageModel now initializes openai.AzureOpenAI
instead of the standard openai.OpenAI client. These Azure-specific kwargs are
kept as client configuration and are not forwarded as chat completions request
parameters.

This preserves the existing OpenAI-compatible base_url path when Azure kwargs
are not provided. It also allows Azure OpenAI authentication with either an API
key, an Azure AD token, or an Azure AD token provider.

How Has This Been Tested?

$ uv run --extra openai --extra test pytest tests/inference_test.py::TestOpenAILanguageModel -q
$ uv run --extra dev pyink langextract/providers/openai.py tests/inference_test.py --check --diff --config pyproject.toml
$ uv run --extra dev pylint --rcfile=.pylintrc langextract/providers/openai.py
$ uv run --extra dev pylint --rcfile=tests/.pylintrc tests/inference_test.py

Additional local verification: I installed langextract from this PR branch in
a downstream sample project and ran an Azure OpenAI extraction smoke check,
verifying a successful Azure OpenAI response with exact text alignment.

Checklist:

• I have read and acknowledged Google's Open Source
  Code of conduct.
• I have read the
  Contributing
  page, and I either signed the Google
  Individual CLA
  or am covered by my company's
  Corporate CLA.
• I have discussed my proposed solution with code owners in the linked
  issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked
  issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes
• I have followed
  Google's Python Style Guide
  and ran pylint over the affected code.

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