Allow BotSharp to run rule-only agents without requiring LLM

[JoPetra]

When creating a rule-based agent (with no LLM configured) using the API, the framework still attempts to resolve an LLM model during conversation.

the output of this line (var settings = settingsService.GetSetting(provider, model);) is null

to generate the issue:

• remove all LLM models from appsettings.json in WebStarter as below

  "LlmProviders": [

  ],