Revert samples/Directory.Packages.props to use preview version until package is published

Co-authored-by: YunchuWang <[email protected]>

Author: Copilot

samples/Directory.Packages.props

@@ -11,7 +11,7 @@
   <!-- Durable sample packages -->
   <ItemGroup>
     <PackageVersion Include="Microsoft.DurableTask.Client.Grpc" Version="1.5.0" />
-    <PackageVersion Include="Microsoft.DurableTask.Generators" Version="1.0.0" />
+    <PackageVersion Include="Microsoft.DurableTask.Generators" Version="1.0.0-preview.1" />
     <PackageVersion Include="Microsoft.DurableTask.Worker.Grpc" Version="1.5.0" />
   </ItemGroup>