fix: use createAzure in validate-model and fix resourceName fallback (fixes #819)

Two related bugs caused Azure OpenAI to pass validation but fail in actual use:

1. validate-model/route.ts used createOpenAI for Azure, while ai-providers.ts
   uses createAzure. These differ in URL construction and authentication headers
   (api-key vs Authorization: Bearer), so validation did not test the real code
   path. Switch to createAzure for consistency.

2. In ai-providers.ts, when a user provided their own API key without a custom
   base URL, resourceName was unconditionally set to undefined. This left
   createAzure with no endpoint information, resulting in resource not found.
   resourceName is an endpoint component, not a credential, so it is safe to
   fall back to AZURE_RESOURCE_NAME from the environment whenever no baseURL is
   available.

app/api/validate-model/route.ts

@@ -4,6 +4,7 @@ import { createDeepSeek, deepseek } from "@ai-sdk/deepseek"
 import { createGateway } from "@ai-sdk/gateway"
 import { createGoogleGenerativeAI } from "@ai-sdk/google"
 import { createVertex } from "@ai-sdk/google-vertex"
+import { createAzure } from "@ai-sdk/azure"
 import { createOpenAI } from "@ai-sdk/openai"
 import { createOpenRouter } from "@openrouter/ai-sdk-provider"
 import { generateText } from "ai"
@@ -126,11 +127,11 @@ export async function POST(req: Request) {
             }
 
             case "azure": {
-                const azure = createOpenAI({
+                const azureClient = createAzure({
                     apiKey,
-                    baseURL: baseUrl,
+                    ...(baseUrl && { baseURL: baseUrl }),
                 })
-                model = azure.chat(modelId)
+                model = azureClient(modelId)
                 break
             }