fix: customize user-agent sent via macOS updater path

[deepak1556]

Fixes #288463

Based on the logs #288463 (comment) the headers from the auto updater client are having the expected Darwin/<kernel version> format. So I am unsure why the user got updates. However, in this change I am updating all our request sites on the macOS path with defined user agent that we control along with some logging.

[Copilot]

Pull request overview

Updates the macOS (Darwin) update flow to send a controlled User-Agent (including Darwin kernel version) across all update request paths, improving update-server EOL detection and adding diagnostics around update checks.

Changes:

• Add getUpdateRequestHeaders() helper to generate macOS update request headers with a custom User-Agent.
• Pass custom headers into Electron auto-updater feed configuration (setFeedURL) and into the manual “no auto-download” update check path.
• Add additional auto-updater event wiring (checking-for-update) and extra tracing/logging around update checks and responses.

Reviewed changes

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

File	Description
src/vs/platform/update/electron-main/updateService.darwin.ts	Apply custom headers to Electron autoUpdater feed, add metered “no-download” request headers, and add additional update-check logging/events.
src/vs/platform/update/electron-main/abstractUpdateService.ts	Introduce shared macOS update header builder and apply it to isLatestVersion() request path, with extra tracing.