fix: assume http/https scheme for scheme-less proxy env vars

[travisbreaks]

Summary

Fixes #4736.

When HTTP_PROXY or HTTPS_PROXY environment variables (or the httpProxy/httpsProxy constructor options) are set without a URL scheme (e.g. localhost:8080 instead of http://localhost:8080), EnvHttpProxyAgent now prepends the appropriate scheme before passing the value to ProxyAgent.

• HTTP_PROXY / httpProxy values default to http://
• HTTPS_PROXY / httpsProxy values default to https://
• Values that already include a scheme (e.g. http://, https://, socks5://) are unchanged

This matches the behavior of curl and Go's httpproxy package, which both assume http:// for scheme-less proxy values. The split behavior (http:// for HTTP_PROXY, https:// for HTTPS_PROXY) was requested by a maintainer in #4860 (comment) and aligns with what curl does.

Changes

• Added normalizeProxyUrl() helper in lib/dispatcher/env-http-proxy-agent.js
• Applied normalization to both env var and constructor option code paths
• Added 6 new test cases covering scheme-less values for env vars, options, hostname-only values, and the no-op case for URLs that already have a scheme

Test plan

• All 40 tests in test/env-http-proxy-agent.js pass (34 existing + 6 new)
• Lint passes

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.91%. Comparing base (51fd661) to head (806eaed).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4914      +/-   ##
==========================================
- Coverage   92.92%   92.91%   -0.01%     
==========================================
  Files         112      112              
  Lines       35646    35668      +22     
==========================================
+ Hits        33123    33141      +18     
- Misses       2523     2527       +4     

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