Handle missing nightly version for --include

Improve CLI package update behavior when resolving the latest `Volo.Abp.Core` nightly version fails or returns empty: the `--include` pass is now skipped explicitly with a warning while normal project updates continue. Also switch file reading in `VoloNugetPackagesVersionUpdater` to use the shared `DefaultEncoding` instead of `Encoding.Default` for consistent encoding handling.

framework/src/Volo.Abp.Cli.Core/Volo/Abp/Cli/ProjectModification/PackagePreviewSwitcher.cs

@@ -273,7 +273,15 @@ public class PackagePreviewSwitcher : ITransientDependency
             // (source registration / regular PackageReference updates below must still
             // proceed for every solution/project) - just skip the --include pass.
             Logger.LogWarning(ex, "Could not resolve the latest Volo.Abp.Core nightly version; --include files will be skipped for this run.");
-            latestVersionFromMyGet = null;
+            return (includeFiles, excludedPackages, null);
+        }
+
+        if (latestVersionFromMyGet.IsNullOrWhiteSpace())
+        {
+            // No exception was thrown, but MyGet simply has no version for this package yet
+            // (e.g. not indexed there) - warn so users aren't left wondering why --include did nothing.
+            Logger.LogWarning("Could not resolve the latest Volo.Abp.Core nightly version; --include files will be skipped for this run.");
+            return (includeFiles, excludedPackages, null);
         }
 
         return (includeFiles, excludedPackages, latestVersionFromMyGet);

framework/src/Volo.Abp.Cli.Core/Volo/Abp/Cli/ProjectModification/VoloNugetPackagesVersionUpdater.cs

@@ -408,7 +408,7 @@ public class VoloNugetPackagesVersionUpdater : ITransientDependency
         {
             using (var fs = File.Open(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None))
             {
-                using (var sr = new StreamReader(fs, Encoding.Default, true))
+                using (var sr = new StreamReader(fs, DefaultEncoding, true))
                 {
                     var fileContent = await sr.ReadToEndAsync();