[Nexthop] Enable "led_service" in FBOSS distro image

[marif-nexthop]

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

led_service was missing from the FBOSS OSS distro image.

In this PR:
Build the led_service binary as part of the FBOSS platform stack and set up the runtime config/settings so it starts on boot:

• Add led_service to the fboss_platform_services CMake target so it ships in platform-stack.tar.

• Add a systemd unit (led_service.service) ordered after platform_manager and fboss_init.

• Enable the unit in the FBOSS distro image (config.sh).

• Add a minimal led.conf at default_configs/ and symlink it from other platforms (minipack3, wedge800bact, wedge800cact and wedge800bnhp etc.).

• Have fboss_init.sh copy /etc/coop/led.conf from the platform's default_configs at boot.

• FSDB subscription paths - OSS stubs in FsdbSwitchStateSubscriber.cpp returned empty path vectors for both switch-state and transceiver-state, so the second addStatePathSubscription({}) collided with the first on the root path ::1:/Path:/state:/ and threw already exists error. Filled the stubs in with the real paths, agent().switchState() andqsfp_service().state().tcvrStates() matching what each callback deserializes which finally fixed the error and led_service could run successfully.

Test Plan

Verified led_service is healthy on a device after loading an image built from this branch.

root        1529  0.0  0.1 7082576 54460 ?       Ssl  18:16   0:01 /opt/fboss/bin/led_service

● led_service.service - FBOSS LED Service
     Loaded: loaded (/usr/lib/systemd/system/led_service.service; enabled; preset: disabled)
     Active: active (running) since Thu 2026-05-28 18:16:33 UTC; 1h 15min ago