[BUG] GenAIModel.cs should not be directly referenced in non-GenAI samples.

[azchohfi]

Describe the bug
Right now, many samples reference GenAIModel.cs, even though we don't specifically need that class.
That is mostly because of some constants that exist there, that should be abstracted into the metadata of IChatClient.
That also forces PhiSilica samples to reference OnnxRuntimeGenAI nugets, which should not be the case at all.

To Reproduce
Steps to reproduce the behavior:

1. Export, for example, the Generate Sample, selecting the PhiSilica API

Expected behavior
It should not reference GenAIModel.cs, and it should not reference OnnxRuntimeGenAI.