Add missing path to verify-samples run checkout

[westey-m]

Motivation and Context

Description

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 dotnet-verify-samples GitHub Actions workflow so the verify-samples job checks out the repository paths needed to execute .NET samples that depend on files outside dotnet/ (notably declarative workflow YAML under declarative-agents/).

Changes:

• Adjusts the workflow’s sparse-checkout paths to include declarative-agents/ (and python/) instead of the non-existent root workflow-samples folder.