Add support for interactive Entra ID authentication to chat_azure().

This commit adds support for another major Azure authentication
approach: the OAuth authorization code flow, as used by the Azure CLI.

This is a good choice for authentiation during development on desktop,
and Microsoft recommends it for Azure OpenAI because it doesn't require
storing sensitive long-lived secrets like API keys.

All of this is pretty stock httr2 OAuth stuff, despite the fact that
Entra ID has its own... idiosyncrasies. I also went out of the way to
add a really specific error message for what I believe to be a common
source of problems: misconfiguration of Azure's RBAC. It looks as
follows:

    Error in `req_perform_connection()` at elmer/R/httr2.R:36:3:
    ! HTTP 401 Unauthorized.
    • PermissionDenied: Principal does not have access to API/Operation.
    ℹ Your user or service principal likely needs one of the following
      roles: Cognitive Services OpenAI User, Cognitive Services OpenAI
      Contributor, or Cognitive Services Contributor.

I haven't added any unit tests (I don't know how to do so for this kind
of interactive OAuth flow), but at least the help documentation has been
updated.

Signed-off-by: Aaron Jacobs <aaron.jacobs@posit.co>

NEWS.md

@@ -37,6 +37,9 @@
 * The `token` argument to `chat_azure()` has been deprecated. Use ambient
   credentials or the `credentials` argument instead (#257, @atheriel).
 
+* `chat_azure()` attempts to use interactive Entra ID authentication if no other
+  credentials are available (#273, @atheriel).
+
 # ellmer 0.1.0
 
 * New `chat_vllm()` to chat with models served by vLLM (#140).

R/provider-azure.R

@@ -18,6 +18,10 @@ NULL
 #' `AZURE_TENANT_ID`, `AZURE_CLIENT_ID`, and `AZURE_CLIENT_SECRET` environment
 #' variables are set.
 #'
+#' Finally, in interactive sessions it will also attempt to use Microsoft Entra
+#' ID authentication -- much like the Azure CLI -- if no API key has been
+#' provided.
+#'
 #' @param endpoint Azure OpenAI endpoint url with protocol and hostname, i.e.
 #'  `https://{your-resource-name}.openai.azure.com`. Defaults to using the
 #'   value of the `AZURE_OPENAI_ENDPOINT` envinronment variable.
@@ -137,7 +141,23 @@ method(chat_request, ProviderAzure) <- function(provider,
   req <- req_retry(req, max_tries = 2)
   req <- req_error(req, body = function(resp) {
     error <- resp_body_json(resp)$error
-    paste0(error$code, ": ", error$message)
+    msg <- paste0(error$code, ": ", error$message)
+    # Try to be helpful in the (common) case that the user or service
+    # principal is missing the necessary role.
+    # See: https://learn.microsoft.com/en-us/azure/ai-services/openai/how-to/role-based-access-control
+    if (error$message == "Principal does not have access to API/Operation.") {
+      msg <- c(
+        "*" = msg,
+        "i" = cli::format_inline(
+          "Your user or service principal likely needs one of the following
+        roles: {.emph Cognitive Services OpenAI User},
+        {.emph Cognitive Services OpenAI Contributor}, or
+        {.emph Cognitive Services Contributor}.",
+          keep_whitespace = FALSE
+        )
+      )
+    }
+    msg
   })
 
   messages <- compact(unlist(as_json(provider, turns), recursive = FALSE))
@@ -214,6 +234,31 @@ default_azure_credentials <- function(api_key = NULL, token = NULL) {
     return(function() list())
   }
 
+  # Masquerade as the Azure CLI.
+  client_id <- "04b07795-8ddb-461a-bbee-02f9e1bf7b46"
+  if (is_interactive() && !is_hosted_session()) {
+    client <- oauth_client(
+      client_id,
+      token_url = "https://login.microsoftonline.com/common/oauth2/v2.0/token",
+      secret = "",
+      auth = "body",
+      name = paste0("ellmer-", client_id)
+    )
+    return(function() {
+      token <- oauth_token_cached(
+        client,
+        oauth_flow_auth_code,
+        flow_params = list(
+          auth_url = "https://login.microsoftonline.com/common/oauth2/v2.0/authorize",
+          scope = "https://cognitiveservices.azure.com/.default offline_access",
+          redirect_uri = "http://localhost:8400",
+          auth_params = list(prompt = "select_account")
+        )
+      )
+      list(Authorization = paste("Bearer", token$access_token))
+    })
+  }
+
   if (is_testing()) {
     testthat::skip("no Azure credentials available")
   }