.NET: Updated Copilot SDK to the latest version

[dmytrostruk]

Motivation and Context

Resolves: #4379

Updated GitHub.Copilot.SDK to 0.1.29 and resolved compatibility with the latest SDK changes.

• Bumped the central package version in Directory.Packages.props.
• Updated Copilot unit tests to use PermissionRequestHandler (replacing the old PermissionHandler delegate usage).

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

Updates the .NET GitHub Copilot adapter’s dependency baseline to align with the latest GitHub.Copilot.SDK, addressing the binary-breaking permission handler change that caused runtime MissingMethodExceptions.

Changes:

• Bump central package version for GitHub.Copilot.SDK to 0.1.29.
• Update Copilot unit tests to use PermissionRequestHandler in place of the previous handler type.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
dotnet/Directory.Packages.props	Updates the centrally-managed GitHub.Copilot.SDK version to 0.1.29.
dotnet/tests/Microsoft.Agents.AI.GitHub.Copilot.UnitTests/GitHubCopilotAgentTests.cs	Adjusts unit tests to match the updated SDK permission request handler API.