Sub Issue 1: Security hardening - SSL bypass, HttpClient pooling, HTTP auth abstraction

[JusterZhu]

Scope

\VersionService, \DownloadManager, \Environments\ + HTTP auth layer

Changes

1. Fix SSL bypass: Remove \CheckValidationResult\ returning always \ rue, replace with standard cert validation + \ISslValidationPolicy\ interface
2. HttpClient pooling: Use \IHttpClientFactory\ instead of
   ew HttpClient()\ every request
3. Optional certificate pinning support
4. \IHttpAuthProvider\ abstraction: Bearer Token / API Key / HMAC-SHA256 pluggable auth

Closes: This is Sub Issue 1 of the GeneralUpdate v2 refactor plan.