This PR addresses an issue that several users have reported. If the
local oauth login server in one codex instance is left running (e.g. the
user abandons the oauth flow), a subsequent codex instance will receive
an error when attempting to log in because the localhost port is already
in use by the dangling web server from the first instance.
This PR adds a cancelation mechanism that the second instance can use to
abort the first login attempt and free up the port.
17a80d43c8