Vendure vulnerable to timing attack that enables user enumeration in NativeAuthenticationStrategy

Vendure is an open-source headless commerce platform. Prior to version 3.5.3, the `NativeAuthenticationStrategy.authenticate()` method is vulnerable to a timing attack that allows attackers to enumerate valid usernames (email addresses). In `packages/core/src/config/auth/native-authentication-strategy.ts`, the authenticate method returns immediately if a user is not found. The significant timing difference (~200-400ms for bcrypt vs ~1-5ms for DB miss) allows attackers to reliably distinguish between existing and non-existing accounts. Version 3.5.3 fixes the issue.