Support Azure OpenAI provider authentication with Azure AD tokens

[DougTrajano]

Request: add support for the Azure OpenAI provider using Azure AD token-based authentication.

Current gap

• The Azure OpenAI integration appears to rely on API key authentication.
• Some deployments require Azure AD auth instead, using tenant ID, client ID, and client secret to obtain access tokens.

Requested behavior

• Allow configuring Azure OpenAI with Azure AD credentials:
  • tenant_id
  • client_id
  • client_secret
• Use those credentials to request and refresh an Azure AD access token automatically.
• Keep existing API key-based auth working if already supported.

Why this matters

• Many enterprise Azure environments prefer or require Azure AD authentication over static API keys.
• This would make the provider usable in more locked-down tenant setups.

Acceptance criteria

• Azure OpenAI requests can authenticate using Azure AD token credentials.
• Token acquisition and refresh are handled transparently.
• Configuration is documented with clear environment variables or settings examples.
• Existing behavior for non-Azure or API key auth is not broken.

If helpful, I can also provide a minimal config example or repro details from our environment.