Add compareBranch to TitleAndDescriptionProvider for enhanced PR context

[shaypet]

Adds compareBranch to the context passed to TitleAndDescriptionProvider.provideTitleAndDescription, and include it in the prompt sent to the model when generating a PR title and description.

Previously the model had no knowledge of which branch is merged, causing it to hallucinate branch names in the generated description. With this change, the user message sent to the model now includes: If merged brand contain X then Y.

Changes

• githubPullRequest.d.ts: Added compareBranch? to the TitleAndDescriptionProvider context interface.
• githubPullRequestTitleAndDescriptionGenerator.ts: Passes compareBranch through to the prompt renderer.
• pullRequestDescriptionPrompt.tsx: Renders the branch name as a sentence at the start of the user message when either value is present.

Part of microsoft/vscode-pull-request-github#8662

copied pr from microsoft/vscode-copilot-chat#5083

Originally, the base branch was also exposed, but this feature was declined by the maintainers of the pull request extension repository.

[Copilot]

Pull request overview

This PR extends the GitHub PR title/description generation prompt context in the Copilot extension by adding the source/compare branch name, so the model can reference the correct branch instead of hallucinating one.

Changes:

• Extends TitleAndDescriptionProvider.provideTitleAndDescription context with compareBranch?: string (and aligns the type to also include template?: string).
• Threads compareBranch through GitHubPullRequestTitleAndDescriptionGenerator into the prompt renderer.
• Updates the PR description prompt to include the compare branch at the start of the user message when present.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.

File	Description
extensions/copilot/src/extension/prompts/node/github/pullRequestDescriptionPrompt.tsx	Adds compare branch text to the generated user message sent to the model.
extensions/copilot/src/extension/prompt/node/githubPullRequestTitleAndDescriptionGenerator.ts	Passes compareBranch through to prompt rendering.
extensions/copilot/src/extension/githubPullRequest.d.ts	Updates the public provider context type to include compareBranch (and template).