fix: auto-detect HTTP proxy tunneling

[mcollina]

This relates to...

Fixes #5093

Rationale

EnvHttpProxyAgent currently inherits ProxyAgent's tunneling behavior. For plain HTTP targets reached through an HTTP proxy, defaulting to CONNECT can break proxies that do not implement tunneling and can lead to the looping behavior reported in #5093.

Changes

• auto-detect whether tunneling is required in ProxyAgent
• keep tunneling enabled whenever either the proxy or the target endpoint is secure
• keep plain HTTP over HTTP proxy requests non-tunneled by default, while preserving proxyTunnel: true as an explicit override
• add regression coverage for EnvHttpProxyAgent using http_proxy
• update the ProxyAgent docs and type comment to describe the new behavior

Features

N/A

Bug Fixes

• fix default proxy behavior for plain HTTP over HTTP proxy connections
• fix the EnvHttpProxyAgent case behind Use of HTTP_PROXY & NODE_USE_ENV_PROXY fails with an infinite loop #5093 by avoiding CONNECT for that setup unless explicitly requested

Breaking Changes and Deprecations

None

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[trivikr]

Should proxyTunnel be set to true on line 76 in test/env-http-proxy-agent-nodejs-bundle.js?

-    setGlobalDispatcher(new EnvHttpProxyAgent())
+    setGlobalDispatcher(new EnvHttpProxyAgent({ proxyTunnel: true }))

[mvolz]

Thanks for submitting this!

Do we have a sense of when this might make it into to a published branch? The reason I ask is that we're trying to replace request with fetch, but our outbound proxy rejects CONNECT requests for https so this is a blocker... trying to decide whether to revert all of it and go back to request we're in a deployable state, or if it's reasonable to wait for this change.