Test PR template support in auto-generated descriptions

[kycutler]

Refs: #5683

• anyOS @ulugbekna
• anyOS @kieferrm

Complexity: 3

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When there is a PR template in the target repository, and the setting "githubPullRequests.pullRequestDescription": "Copilot" is enabled, opening a PR via the extension should auto-generate the PR description based on the template.

[ulugbekna]

reference how to add a PR template to a repo - https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository