.NET - [Breaking]: Update Declarative Object Model + Dependencies

[crickman]

Motivation and Context

Microsoft.Bot.ObjectModel.* packages and namespaces have been updated to Microsoft.Agents.ObjectModel.*

Note: Requires nuget.org to be update in order to build successfully in github.

Fixes: #3018

Description

This change updates the dependencies on the Microsoft.Bot.ObjectModel.* packages to the to Microsoft.Agents.ObjectModel.* packages and also picks up an update that includes media type for image or file based message content.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR updates dependencies from the deprecated Microsoft.Bot.ObjectModel.* packages to the new Microsoft.Agents.ObjectModel.* packages (version 1.2025.1222.3), and refactors the message content type schema to support media type information for image and file-based content.

Key Changes:

• Renamed namespace from Microsoft.Bot.ObjectModel to Microsoft.Agents.ObjectModel across all files
• Restructured TypeSchema to separate message content fields into a dedicated MessageContent class
• Updated field names: ContentType/ContentValue → Type/Value with new MediaType field support
• Added new NuGet package source for the Microsoft.Agents.ObjectModel packages

Reviewed changes

Copilot reviewed 160 out of 160 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
dotnet/Directory.Packages.props	Updated package references to Microsoft.Agents.ObjectModel.* version 1.2025.1222.3
dotnet/nuget.config	Added CopilotStudio package source for Microsoft.Agents.ObjectModel packages
dotnet/src/**/*.csproj	Updated PackageReference from Microsoft.Bot.ObjectModel to Microsoft.Agents.ObjectModel
dotnet/src/**/*.cs	Updated using statements from Microsoft.Bot.ObjectModel to Microsoft.Agents.ObjectModel
PowerFx/TypeSchema.cs	Refactored schema: extracted MessageContent class with Type/Value/MediaType fields
Extensions/ChatMessageExtensions.cs	Updated to use new TypeSchema.MessageContent field names and added MediaType support
PowerFx/Functions/MessageFunction.cs	Updated to use TypeSchema.MessageContent for content fields
dotnet/tests/**/*Tests.cs	Updated test assertions to use new TypeSchema.MessageContent field names