refactor: replace string formatting with url.URL to support ipv6

[zzy2210]

#179 179

[Copilot]

Pull Request Overview

This PR refactors the endpoint URL construction to use Go's url.URL type instead of string formatting for better URL handling and IPv6 compatibility.

• Replaces fmt.Sprintf with structured URL building using url.URL
• Uses net.JoinHostPort for proper host:port formatting that handles IPv6 addresses
• Removes dependency on the fmt package

Comments suppressed due to low confidence (1)

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}