fix: normalize double slashes in URI building

[juandiii]

Summary

Fixes URI concatenation when baseUrl ends with / and path starts with /, causing double slashes (//) that result in 301 redirects.

Problem

// baseUrl: "https://api.ipify.org/"
// @HttpExchange("/")
// Result: https://api.ipify.org//  ← double slash → 301 Moved Permanently

Solution

Normalize duplicate slashes in AbstractClientRequest.buildUri() while preserving https://.

Tests

6 new test cases covering:

• Trailing slash on baseUrl + leading slash on path
• Empty path
• Multiple consecutive slashes
• Protocol slashes preserved