Copilot Chat setup script fixes

[gitri-ms]

Motivation and Context

• Chocolatey won't install if %ProgramData%\Chocolatey exists but is empty
• Requirement to specify tenant is confusing, and not actually necessary
• Instructions for editing appsettings.json are unclear & easy to miss

Description

• Check for choco.exe instead of just checking for install directory
• Remove requirement to specify tenant in Setup.ps1 and Setup.sh. This aligns with the full Copilot Chat setup instructions, which just use the common endpoint and don't prompt the user for this.
• Add appsettings.json to bulleted list of prerequisites for running these scripts

Contribution Checklist

• [x] The code builds clean without any errors or warnings
• [x] The PR follows SK Contribution Guidelines (https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
• [x] The code follows the .NET coding conventions (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions) verified with dotnet format
• [x] All unit tests pass, and I have added new tests where possible
• [x] I didn't break anyone :smile: