Add compareBranch to TitleAndDescriptionProvider for enhanced PR context (#312326)

* feat: add compareBranch to TitleAndDescriptionProvider for enhanced PR context

* feat: enhance PR description to include escaped compare branch name

* Update extensions/copilot/src/extension/prompts/node/github/pullRequestDescriptionPrompt.tsx

Co-authored-by: Alex Ross <38270282+alexr00@users.noreply.github.com>

* feat: update pull request user message to include escaped compare branch name

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Co-authored-by: Alex Ross <38270282+alexr00@users.noreply.github.com>