.NET: feat: Update Github Copilot SDK to 1.0.0-beta.2

[lokitoth]

Motivation and Context

The current GH Copilot SDK reference is multiple months old, and is also referencing a package triggering a NuGet warning-as-error on build.

• 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.

[github-actions]

Automated Code Review

Reviewers: 4 | Confidence: 84% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Design Approach

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Automated review by lokitoth's agents

[Copilot]

Pull request overview

Updates the .NET GitHub Copilot integration to use GitHub.Copilot.SDK 1.0.0-beta.2, including a small API adaptation to the SDK’s attachment model.

Changes:

• Bump GitHub.Copilot.SDK from 0.1.29 to 1.0.0-beta.2 via central package management.
• Update GitHubCopilotAgent to use the new UserMessageAttachmentFile type for DataContent-backed attachments.
• Minor formatting change in a unit test file.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.

File	Description
dotnet/Directory.Packages.props	Updates the centralized NuGet version for GitHub.Copilot.SDK to 1.0.0-beta.2.
dotnet/src/Microsoft.Agents.AI.GitHub.Copilot/GitHubCopilotAgent.cs	Adapts attachment creation to the new SDK attachment type used when sending messages.
dotnet/tests/Microsoft.Agents.AI.GitHub.Copilot.UnitTests/GitHubCopilotAgentTests.cs	Contains a whitespace-only change in the CopySessionConfig_CopiesAllProperties test.

[rogerbarreto]

Force merge to avoid the dead lock with the hosted-it failure which fix is also locked out due to the vulnerability.