Update configuration error messages for Endpoint and ApiKey

Refactor the retrieval of `Endpoint` and `ApiKey` from the application configuration.
The error messages have been made more specific by changing the context from "AzureOpenAi:Endpoint" to "Endpoint" and from "AzureOpenAi:ApiKey" to "ApiKey".
This improves clarity in error reporting when these configuration values are missing.

Author: EdCharbeneau

common/microsoft-extensions-ai-integration/AIPromptIntegration/AIPromptIntegration/Program.cs

@@ -25,9 +25,9 @@
 //   dotnet user-secrets set ApiKey super-secret-api-key
 
 // 🌐 The Uri of your provider
-var endpoint = builder.Configuration["Endpoint"] ?? throw new InvalidOperationException("Missing configuration: AzureOpenAi:Endpoint. See the README for details.");
+var endpoint = builder.Configuration["Endpoint"] ?? throw new InvalidOperationException("Missing configuration: Endpoint. See the README for details.");
 // 🔑 The API Key for your provider
-var apikey = builder.Configuration["ApiKey"] ?? throw new InvalidOperationException("Missing configuration: AzureOpenAi:ApiKey. See the README for details.");
+var apikey = builder.Configuration["ApiKey"] ?? throw new InvalidOperationException("Missing configuration: ApiKey. See the README for details.");
 // 🧠 The model name or azure deployment name
 var model = "YOUR_MODEL_NAME";