fix(vscode): support workspace review before first commit

[LKL-ZREO]

Description

Workspace review currently fails in a newly initialized Git repository before its first commit. git diff --name-status HEAD cannot resolve an unborn HEAD; because it runs inside Promise.all, the rejection also discards the successfully collected untracked files and clears workspaceFiles. As a result, the VS Code extension disables starting a review even when the repository contains staged or untracked changes.

This change:

• treats a failed HEAD diff as unavailable and reuses the existing cached diff fallback;
• continues to include untracked files in workspace review;
• preserves the existing behavior for repositories with commits;
• aligns VS Code workspace discovery with the CLI's existing fallback semantics;
• adds regression coverage using a real Git repository without any commits.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (no functional changes)
• Documentation update
• CI / Build / Tooling

How Has This Been Tested?

• make test passes locally
• Manual testing (describe below)

Automated checks:

• yarn install --frozen-lockfile
• yarn lint
• yarn compile
• yarn build
• Regression test for staged and untracked files before the first commit
• All other VS Code test suites pass locally

The full test run on Windows has one pre-existing failure in
CliService.isAvailable related to spawning npm; this is unrelated to this
change and is being addressed by #490. The new regression test passes, and the
remaining 88 tests pass locally.

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have added tests that prove my fix is effective
• I have updated the documentation accordingly (not applicable: no user-facing API or workflow change)
• I have signed the CLA

Related Issues

N/A. Follow-up to #334.

[CLAassistant]

All committers have signed the CLA.

[github-actions]

✅ OpenCodeReview: No comments generated. Looks good to me.

[lizhengfeng101]

LGTM