Passing a custom transport disables environment proxies

[RobertCraigie]

This behaviour was added in #1122 and I'm curious why transport was included?

In anthropic-python we recently released an update that includes sending some TCP keep alive packets but incidentally this also broke environment proxies.

Additionally, it looks like the code used to read these environment variables is all private? How would we support both environment variables & a custom transport? Would we have to essentially duplicate the logic in _get_proxy_map() and pass it to mounts?