fix: preserve base path when constructing API URLs in GBContext

[vazarkevych]

Summary

• Fixes a bug where getFeaturesURL and getRemoteEvalUrl would drop any sub-path from hostURL (e.g. https://something.com/some-other-path would produce https://something.com/api/... instead of https://something.com/some-other-path/api/...)
• Extracted shared logic into a private _buildUrl helper to eliminate duplication
• Added trailing slash normalization to handle multiple consecutive slashes in hostURL