.Net: Added missing KernelFunction attribute to DownloadFileAsync in SessionsPythonPlugin (#11215)

### Description

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Added missing KernelFunction attribute to DownloadFileAsync in
SessionsPythonPlugin

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- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
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raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

Author: Krzysztof318

dotnet/src/Plugins/Plugins.Core/CodeInterpreter/SessionsPythonPlugin.cs

@@ -47,7 +47,7 @@ public SessionsPythonPlugin(
 
         this._settings = settings;
 
-        // Ensure the endpoint won't change by reference 
+        // Ensure the endpoint won't change by reference
         this._poolManagementEndpoint = GetBaseEndpoint(settings.Endpoint);
 
         this._authTokenProvider = authTokenProvider;
@@ -184,7 +184,7 @@ public async Task<SessionsRemoteFileMetadata> UploadFileAsync(
     /// <param name="remoteFilePath"> The path to download the file from, relative to `/mnt/data`. </param>
     /// <param name="localFilePath"> The path to save the downloaded file to. If not provided won't save it in the disk.</param>
     /// <returns> The data of the downloaded file as byte array. </returns>
-    [Description("Downloads a file from the current Session ID.")]
+    [KernelFunction, Description("Downloads a file from the current Session ID.")]
     public async Task<byte[]> DownloadFileAsync(
         [Description("The path to download the file from, relative to `/mnt/data`.")] string remoteFilePath,
         [Description("The path to save the downloaded file to. If not provided won't save it in the disk.")] string? localFilePath = null)