fix(cmd): preserve user-supplied base-url in ucloud init

init unconditionally reset ConfigIns.BaseURL to DefaultBaseURL right
before persisting, discarding the custom base-url entered at the prompt.
The value only took effect during remote validation and the confirmation
line echoed the overwritten default, so users could not tell their input
was dropped. Dedicated-cloud customers pointing at a non-primary gateway
had to work around it via `ucloud config update --base-url`.

Remove the overwrite; ConfigBaseURL() already leaves BaseURL populated
(the package-level default when the user just presses Enter). Timeout,
MaxRetryTimes and Active still reset to their defaults since init has no
prompt for them.

Add TestInitPersistsUserSuppliedBaseURL, which drives the real
NewCmdInit().Run() against a fake gateway and asserts the reloaded
profile keeps the entered base-url.