fix(chat): replace ex.Message with generic client-safe error in streaming responses

Exception details are now only in server logs (already logged via LogChatStreamErrorBeforeResponseStarted
and LogChatStreamErrorMidStream). The errorCode from ChatBackendUnavailableException is still
forwarded as it is a controlled application-level value.

Author: BenjaminMichaelis

EssentialCSharp.Web/Controllers/ChatController.cs

@@ -278,14 +278,14 @@ public async Task StreamMessage([FromBody] ChatMessageRequest request, Cancellat
             LogChatStreamErrorBeforeResponseStarted(_Logger, ex, User.Identity?.Name);
             Response.StatusCode = StatusCodes.Status503ServiceUnavailable;
             Response.ContentType = "application/json";
-            await Response.WriteAsJsonAsync(new { error = ex.Message, errorCode = ex.ErrorCode }, cancellationToken);
+            await Response.WriteAsJsonAsync(new { error = "Chat service unavailable", errorCode = ex.ErrorCode }, cancellationToken);
         }
         catch (ChatBackendUnavailableException ex)
         {
             LogChatStreamErrorMidStream(_Logger, ex, User.Identity?.Name);
             try
             {
-                var eventData = JsonSerializer.Serialize(new { type = "error", message = ex.Message, errorCode = ex.ErrorCode });
+                var eventData = JsonSerializer.Serialize(new { type = "error", message = "Chat service unavailable", errorCode = ex.ErrorCode });
                 await Response.WriteAsync($"data: {eventData}\n\n", CancellationToken.None);
                 await Response.Body.FlushAsync(CancellationToken.None);
             }