Copilot Chat: Update to newest SK nuget

[stephentoub]

Motivation and Context

Update CopilotChat sample to the latest SK nuget version, absorbing breaking changes. I've done zero testing on this, other than "it builds", so we'll want to validate it before merging.
cc: @shawncal

This cleans up some cruft related to skill definitions, though there's still likely more that can be done for someone more familiar than I am with this code. I expect we'll be able to clean it up further once the exception handling changes land, as there's a lot of code dedicated to checking whether an error has occurred and short-circuiting if one has. (@SergeyMenshykh, this would be a good one to pay attention to, in particular around how exceptions propagate out of semantic functions instead of relying on manual checking of the returned context.)

Contribution Checklist

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

[crickman]

Appreciate the update! Great to keep this in sync to show the current patterns.