[Nexthop][fboss2-dev] Add interface loopback-mode config command

[hillol-nexthop]

Based on #1095

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

1. New config interface attributes — gives operators direct CLI control over port properties previously only configurable by editing switch config JSON.

New config command syntax:

fboss2-dev config interface <port-list> loopback-mode [none|PHY|NIF|MAC]
fboss2-dev config interface <port-list> flow-control-rx [enable|disable]
fboss2-dev config interface <port-list> flow-control-tx [enable|disable]
fboss2-dev config interface <port-list> lldp-expected-value <string>
fboss2-dev config interface <port-list> lldp-expected-chassis <string>
fboss2-dev config interface <port-list> lldp-expected-ttl <string>
fboss2-dev config interface <port-list> lldp-expected-port-desc <string>
fboss2-dev config interface <port-list> lldp-expected-system-name <string>
fboss2-dev config interface <port-list> lldp-expected-system-desc <string>
fboss2-dev config interface <port-list> type routed-port
fboss2-dev config interface <port-list> shutdown
fboss2-dev config interface <port-list> no-shutdown

2. Consolidated delete interface commands — replaces separate per-attribute subcommand classes (CmdDeleteInterfaceLoopbackMode, CmdDeleteInterfaceLldpExpectedValue) with a single CmdDeleteInterface handler, mirroring the CmdConfigInterface pattern.

New delete command syntax:

fboss2-dev delete interface <port-list> loopback-mode
fboss2-dev delete interface <port-list> lldp-expected-value
fboss2-dev delete interface <port-list> lldp-expected-chassis
fboss2-dev delete interface <port-list> lldp-expected-ttl
fboss2-dev delete interface <port-list> lldp-expected-port-desc
fboss2-dev delete interface <port-list> lldp-expected-system-name
fboss2-dev delete interface <port-list> lldp-expected-system-desc
# Multiple attrs in one call:
fboss2-dev delete interface <port-list> loopback-mode lldp-expected-chassis

Implementation

All attributes map to existing fields on cfg::Port — no thrift schema changes.

| Attribute | Thrift field | Notes |
|--------|-----------|-------------|-------|
loopback-mode | cfg::Port.loopbackMode (field 17) | NONE/PHY/NIF/MAC, case-insensitive |
flow-control-rx | cfg::Port.pause.rx (field 13) | enable/disable |
flow-control-tx | cfg::Port.pause.tx (field 13) | enable/disable |
lldp-expected-* | cfg::Port.expectedLLDPValues (field 20) | 6 tag variants |
type routed-port | cfg::Port.portType (field 27) | also sets routable=true, clears ingressVlan |
shutdown / no-shutdown | cfg::Port.state (field 2) | valueless flags |

• New InterfaceDeleteAttrs arg type parses <ports> <attr> [<attr>...] — same pattern as InterfacesConfig but all attributes are valueless (reset-to-default semantics)
• operator const InterfaceList&() implicit conversion preserves the ipv6 nd subcommand chain
• Adds delete support for all 6 lldp-expected-* tag variants (only lldp-expected-value was supported before)
• Removes 4 files (2 old per-attribute classes + their headers), consolidates 4 test files into 2

Test plan

• fboss2-dev config interface eth1/1/1 loopback-mode PHY — sets loopback mode
• fboss2-dev delete interface eth1/1/1 loopback-mode — resets to NONE
• fboss2-dev delete interface eth1/1/1 loopback-mode lldp-expected-chassis — resets both in one call
• fboss2-dev config interface eth1/1/1 shutdown / no-shutdown — disables/enables port
• fboss2-dev delete interface eth1/1/1 ipv6 nd ra-interval — ipv6 nd subcommand still works
• Unit tests: CmdDeleteInterfaceTest (7 cases), CmdConfigInterfaceLoopbackModeTest, etc.