Add chat_azure_anthropic() for Azure Foundry Anthropic Models

[dareneiri]

Summary

A new function chat_azure_anthropic() allows users to access Anthropic models made available through Azure Foundry, which has a different access point from the existing Azure OpenAI implementation in chat_azure_openai()

Azure AI Foundry exposes Anthropic models via a different endpoint format than chat_azure_openai(), which targets *.openai.azure.com and uses the OpenAI chat completions format. This new function targets the Foundry endpoint and uses the standard Anthropic Messages API format (x-api-key, anthropic-version: 2023-06-01), meaning it inherits all existing ProviderAnthropic behaviour — streaming, tool calling, structured data extraction, prompt caching, batch chat, and image/PDF support — with no additional implementation required.

Changes

• New function: chat_azure_anthropic() which connects to Anthropic models hosted at*.services.ai.azure.com/anthropic
• Adds new line to news.md on new function

Implementation details

• ProviderAzureAnthropic inherits from ProviderAnthropic with no additional properties — the Foundry Anthropic endpoint uses the same API format as api.anthropic.com
• model is required with no default, since the value is a user-specific Azure deployment name rather than a canonical model identifier, similar to Azure Open AI
• Trailing slashes on endpoint are normalised to avoid double-slash URLs
• default_azure_anthropic_credentials() mirrors default_azure_credentials() with the same priority order

Testing

• ✅ New test added: tests/testthat/test-provider-azure-anthropic.R

devtools::test(filter = "provider-azure-anthropic")
# [ FAIL 0 | WARN 0 | SKIP 1 | PASS 15 ]
# SKIP: connectcreds not installed (expected)

> chat <- chat_azure_anthropic(model = "claude-sonnet-4-6")
> chat$chat("What is the capital of France?")
The capital of France is **Paris**.

[dareneiri]

I previously wrote that this addresses #585 but that specifically mentions the ability to call Mistral models through Azure Foundry. This new function only concerns Anthropic models. Mistral models will call *.services.ai.azure.com/openai/v1/.