Cache branch names for faster target branch picker in Create PR view

[Copilot]

The target branch picker makes a full GraphQL paginated fetch on every open, causing noticeable delay. Branches should appear instantly on subsequent opens, and pre-fetching should warm the cache before the user even clicks.

Changes

• In-memory branch cache in GitHubRepository: listBranches() caches results for unprefixed (full list) queries. getCachedBranches() provides synchronous retrieval.
• Two-phase quickpick population: changeRemoteAndBranch() now shows cached branch picks immediately, then refreshes in the background with fresh API data.
• Pre-fetch on view init: getCreateParams() fires a non-blocking listBranches() call for the base remote so the cache is warm by the time the user opens the picker.
• Extracted buildBranchPickItems helper in quickPicks.ts shared by both branchPicks() (async, fetches) and cachedBranchPicks() (sync, cache-only).

Flow

Create PR view opens
  └─ getCreateParams() → prefetchBranches() fires in background
                            └─ listBranches() populates _branchesCache

User clicks base branch picker
  └─ cachedBranchPicks() → instant display from cache
  └─ updateItems() → background refresh with fresh API data

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[Copilot]

Pull request overview

Improves responsiveness of the Create PR target/base branch picker by warming and reusing an in-memory branch list cache so the picker can render immediately on subsequent opens.

Changes:

• Add an in-memory unprefixed branch list cache to GitHubRepository with synchronous retrieval.
• Update the Create PR branch picker to populate from cache immediately, then refresh from the API in the background.
• Trigger a non-blocking branch prefetch during Create PR view initialization to warm the cache.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/github/githubRepository.ts	Adds _branchesCache, getCachedBranches(), and populates the cache from listBranches() for unprefixed queries.
src/github/quickPicks.ts	Extracts buildBranchPickItems and adds cachedBranchPicks() for sync, cache-backed pick construction.
src/github/createPRViewProvider.ts	Prefetches branches on view init and uses cached branch picks for immediate quickpick display before background refresh.
src/@types/vscode.proposed.chatSessionsProvider.d.ts	Updates proposed API parameter naming/documentation for registerChatSessionContentProvider.