BFD configuration provided at rack setup time is lost in the handoff to Nexus

When Nexus gets the handoff from RSS, the handoff request includes the full RackNetworkConfig provided by the operator to RSS. Nexus does not destructure that, and instead picks apart individual fields; e.g., setting up the initial address lot: https://github.com/oxidecomputer/omicron/blob/a3012e0080d7aca311e2dfe2f45cba7ae828ca5b/nexus/src/app/rack.rs#L318-L321

and any BGP config: https://github.com/oxidecomputer/omicron/blob/a3012e0080d7aca311e2dfe2f45cba7ae828ca5b/nexus/src/app/rack.rs#L344

However, nowhere in here does it access rack_networking_config.bfd, which I believe means we'd see something like this:

• The RackNetworkConfig as provided by the operator at RSS time would be written to the bootstore during RSS
• Any BFD information included in the RSS config would be used during the RSS network setup, because sled-agent does read this information and apply it in its "early networking" path
• During handoff, Nexus would not persist this BFD config into cockroach
• Therefore, the first time the sync_switch_configuration task ran after RSS, it would construct a new RackNetworkConfig with no BFD configuration at all, and would overwrite the bootstore RackNetworkConfig set up by RSS, as well as actively tearing down any BFD sessions set up during RSS

I ran into this while attempting to test some maghemite BFD changes in a4x2, where following these instructions produced a setup where the ipv6 links had configured BFD peers and the ipv4 links did not, even though the ipv4 links' BFD configs were present in the RSS config toml.

I tried to see if this was lost along the way, but I think maybe it has never worked? #4852 was the initial BFD support PR, but it didn't include RSS or sled-agent support, only configuration via the external API after the rack was set up. #5313 went back and added it to the RSS config, bootstore, and sled-agent, but didn't touch the RSS -> Nexus handoff path.

Start in nexus/src/app/rack.rs at the RSS-to-Nexus handoff, especially where RackNetworkConfig fields and BGP configuration are extracted. Trace how rack_networking_config.bfd should reach bootstore and the sync_switch_configuration task. Done means BFD supplied in RSS configuration is persisted through handoff and is not removed when synchronization runs.