fix: Adjust checkout tasks

[oZakari]

Overview/Summary

This pull request includes updates to the GitHub Actions workflows to improve the validation process for queries and recommendations. The most important changes include modifying the checkout reference for pull requests and adding a sanity check step.

Changes to validation workflows:

• .github/workflows/validate-queries.yml: Modified the checkout reference to use refs/pull/${{ github.event.number }}/merge and added a sanity check step to verify the pull request's head SHA.
• .github/workflows/validate-recommendations.yml: Removed the ref parameter from the checkout step.

Breaking Changes

None

As part of this pull request I have

• Read the Contribution Guide and ensured this PR is compliant with the guide
• Checked for duplicate Pull Requests
• Associated it with relevant GitHub Issues or ADO Work Items (Internal Only)
• Ensured my code/branch is up-to-date with the latest changes in the main branch
• Ensured PR tests are passing
• Performed testing and provided evidence (e.g. screenshot of output) for any changes associated to ARG queries
• Updated relevant and associated documentation (e.g. Contribution Guide, Docs etc.)