cli: restore default value for device interface create --bandwidth

[thijsvanemmerik]

Summary

• --bandwidth on doublezero device interface create became mandatory after PR cli: display bandwidth/cir as human-readable strings, update help text #3077, even though that PR's description stated it was now optional.
• Root cause: PR cli: display bandwidth/cir as human-readable strings, update help text #3077 dropped default_value = "0" from the clap attribute on bandwidth. Because the field is u64 (not Option<u64>), clap then treats omission as a missing required argument. Only cir actually became optional in that PR.
• Restores default_value = "0bps" to match the existing cir attribute on the next line. 0bps is a valid input for validate_parse_bandwidth.

Testing Verification

• Added regression test test_cli_device_interface_create_bandwidth_is_optional that uses clap::try_parse_from without --bandwidth and asserts parsing succeeds with bandwidth == 0.
• Verified the test fails on pre-fix code with clap error MissingRequiredArgument for --bandwidth <BANDWIDTH>, and passes after the fix.
• All 336 doublezero_cli unit tests pass.

[ben-malbeclabs]

I think we need to scope this out a bit more:

• We have bandwidth in two places: links and interfaces
• We want bandwidth on the CYOA interfaces not connected to a link to be able to track capacity
• We want bandwidth for the interface to match the link
• Bandwidth is not an attribute of a loopback interface

We could place a CLI check that prevents:
interface[a|z] bandwidth < link bandwidth

[juan-malbeclabs]

@thijsvanemmerik @ben-malbeclabs If that sounds good to everyone, we can move forward with this change and I can send a follow-up PR with the validations Ben suggested. Then we can merge them in sequence.

[thijsvanemmerik]

Sounds good Juan, thanks for picking up the follow-up. Happy to keep this PR as the minimal loopback regression fix.

Ben, your scoping is right. The two checks sit naturally in different commands: require non-zero bandwidth for CYOA and DIA interfaces at device interface create and device interface update, and enforce interface[a|z].bandwidth >= link.bandwidth at link wan-create / dzx-create.

[juan-malbeclabs]

New PR: #3778

[juan-malbeclabs]

LGTM -> #3778 bandwidth validation