Fail to run GitHub Models samples

**Description**
When testing the [GitHub Models example](https://github.com/Azure-Samples/netaivideoanalyzer?tab=readme-ov-file#run-github-models-samples), after running `dotnet user-secrets set "GITHUB_TOKEN" "< your github personal access tokens>"`, running `dotnet run` fails with the following error:

> Notes:
Besides, the command `dotnet user-secrets set "GITHUB_TOKEN" "< your github personal access tokens>"` to set `GITHUB_TOKEN` is missing from readme.md, please add it.

**Repro steps**
1. Create a new Codespace.
2. Run `cd ./src/ConsoleMEAI-05-GitHubModels/`.
3. Run `dotnet user-secrets set "GITHUB_TOKEN" "< your github personal access tokens>"`.
4. Run `dotnet run`.

**Expected behavior**
GitHub Models samples can be run successfully.

@elbruno and @hemarina for notification.