[Nexthop] fast-reboot: save FDB, ARP and route state for platforms that cold-start SAI after kexec

[wren-nexthop]

What I did

Resolves #4754

Save FDB, ARP/neighbor, default-route and media state to /host/fast-reboot before the fast-reboot kexec, so swss can restore it after boot.

swss startup already restores these snapshots — files/build_templates/docker_image_ctl.j2 copies fdb.json / arp.json / default_routes.json / media_config.json into the swss container before signaling swssconfig — but nothing in the fast-reboot flow produces them anymore. On platforms where fast-reboot re-initializes the ASIC after kexec (no SAI warm-boot state is preserved), the snapshots are the recovery input that shortens post-boot relearning.

How I did it

Add dump_state_for_fast_boot() to scripts/fast-reboot, called for fast-reboot only, right before the point of no return:

• Skips (with a debug log) when fast-reboot-dump.py is not present, and on multi-ASIC platforms.
• Recreates /host/fast-reboot, runs fast-reboot-dump.py -t /host/fast-reboot, and removes the directory again if the dump fails so a partial snapshot is never restored.

How to verify it

1. sudo fast-reboot -v — the log shows Dumping FDB/ARP/route/serdes state to /host/fast-reboot ....
2. Before kexec, /host/fast-reboot contains fdb.json, arp.json, default_routes.json, media_config.json.
3. After boot, swssconfig logs show the JSON files being loaded, and the directory is cleaned up by the existing swss startup logic.

Verified on Broadcom Tomahawk-based platforms whose fast-reboot path cold-starts SAI after kexec.

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[mssonicbld]

/azp run

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