Add base and compare branch names to Copilot PR title and description generation

[shaypet]

Adds baseBranch and compareBranch to the context passed to TitleAndDescriptionProvider.provideTitleAndDescription, and forwards them from all three call sites where title and description generation is triggered.

Previously the model had no knowledge of which branches were involved, causing it to hallucinate branch names in the generated description. With this change, the actual branch names are passed to the provider so they can be included in the prompt.

Changes

• src/api/api.d.ts: Added baseBranch? and compareBranch? to the TitleAndDescriptionProvider context interface.
• src/github/createPRViewProvider.ts: Passes this.model.baseBranch and this.model.compareBranch when calling provideTitleAndDescription from the Create PR view.
• src/github/pullRequestOverview.ts: Passes this._item.base.ref and this._item.head?.ref when regenerating a description from the PR overview.
• src/commands.ts: Passes args.baseBranch and args.compareBranch when calling through the github.createPullRequest.generate command API.

Fixes #8662

The corresponding change to the Copilot Chat extension that uses these values to render them in the prompt is at microsoft/vscode-copilot-chat#5083.

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[Copilot]

Pull request overview

This PR improves PR title/description generation by including the actual base and compare branch names in the context passed to TitleAndDescriptionProvider.provideTitleAndDescription, reducing branch-name hallucinations during generation (fixes #8662).

Changes:

• Extended the TitleAndDescriptionProvider context type to include optional baseBranch and compareBranch.
• Forwarded baseBranch/compareBranch from the Create PR view generation flow.
• Forwarded baseBranch/compareBranch from PR Overview “regenerate description” and from the command-based generation entry point.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
src/api/api.d.ts	Adds optional baseBranch/compareBranch fields to the provider context type.
src/github/createPRViewProvider.ts	Passes the Create PR model’s base/compare branch names into generation context.
src/github/pullRequestOverview.ts	Passes PR base/head refs into generation context when regenerating description.
src/commands.ts	Passes command args base/compare branch names into generation context.

[shaypet]

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