CopilotChat: Make Azure Speech optional on the backend and don't show…

… errors when not configured on the frontend (#652)

### Motivation and Context
Fix for failures when AzureSpeech is not configured.

### Description
Made AzureSpeech options optional and return OK with a token response
that is marked as not successful.

samples/apps/copilot-chat-app/webapi/Config/AzureSpeechOptions.cs

@@ -14,12 +14,10 @@ public sealed class AzureSpeechOptions
     /// <summary>
     /// Location of the Azure speech service to use (e.g. "South Central US")
     /// </summary>
-    [Required, NotEmptyOrWhitespace]
     public string? Region { get; set; } = string.Empty;
 
     /// <summary>
     /// Key to access the Azure speech service.
     /// </summary>
-    [Required, NotEmptyOrWhitespace]
-    public string Key { get; set; } = string.Empty;
+    public string? Key { get; set; } = string.Empty;
 }

samples/apps/copilot-chat-app/webapi/Controllers/SpeechTokenController.cs

@@ -36,9 +36,11 @@ public SpeechTokenController(IOptions<AzureSpeechOptions> options, ILogger<Speec
     [ProducesResponseType(StatusCodes.Status200OK)]
     public async Task<ActionResult<SpeechTokenResponse>> GetAsync()
     {
-        if (string.IsNullOrWhiteSpace(this._options.Region))
+        // Azure Speech token support is optional. If the configuration is missing or incomplete, return an unsuccessful token response.
+        if (string.IsNullOrWhiteSpace(this._options.Region) ||
+            string.IsNullOrWhiteSpace(this._options.Key))
         {
-            throw new InvalidOperationException($"Missing value for {AzureSpeechOptions.PropertyName}:{nameof(this._options.Region)}");
+            return new SpeechTokenResponse { IsSuccess = false };
         }
 
         string fetchTokenUri = "https://" + this._options.Region + ".api.cognitive.microsoft.com/sts/v1.0/issueToken";