lib: add NODE_PROXY_TUNNEL env var to disable CONNECT tunneling

[mcollina]

When NODE_USE_ENV_PROXY or --use-env-proxy is enabled and the proxy does not support the HTTP CONNECT method, undici loops forever retrying CONNECT requests. Add NODE_PROXY_TUNNEL env var to control whether proxy connections use CONNECT tunneling.

When set to false/0, EnvHttpProxyAgent passes proxyTunnel: false to ProxyAgent, which uses Http1ProxyWrapper for direct HTTP forwarding instead of CONNECT tunneling.

Refs: nodejs/undici#5093

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net

[joyeecheung]

IIUC this is what nodejs/undici#4083 is about. Shouldn't this be the default behavior? For http.request, we just rewrite the request instead of tunneling at all to stay aligned with curl's behavior, I think we should do the same for fetch as well and just don't bother tunneling pure HTTP requests.

[joyeecheung]

Does this disable tunneling for HTTPS requests too? If so, that sounds like an insecure behavior - HTTPS should always be tunneled while HTTP doesn't need tunneling. That's what curl has and also what http/https.request implements. If it only disables tunneling for HTTP, I think we can just unconditionally set it to false and don't need to add an env var for this unless anyone specifically requests being able to tunnel pure HTTP requests (I doubt anyone would actually need it considering that's not what curl and most other runtimes do)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.65%. Comparing base (c3dd52a) to head (4b0007d).
⚠️ Report is 83 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62926      +/-   ##
==========================================
+ Coverage   89.61%   89.65%   +0.03%     
==========================================
  Files         706      706              
  Lines      219136   219375     +239     
  Branches    41981    42055      +74     
==========================================
+ Hits       196376   196675     +299     
+ Misses      14671    14592      -79     
- Partials     8089     8108      +19     

Files with missing lines	Coverage Δ
lib/internal/process/pre_execution.js	98.27% <100.00%> (-0.12%)	⬇️

... and 77 files with indirect coverage changes

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