Replace switch backend variables with tagged deployment enum

[sion42x]

Changes based on feedback in #11237. This method drastically simplifies the tracking of switch backend by sled-agent and improves downstream understanding by naming the types of configs. It also means no startup probe is run when production option (tofino) is chosen which is more in line with the current method. Current checked in configs are production, virtual, standalone, and custom.

[sion42x]

@jmpesp I say old chap this does feel cleaner

https://github.com/oxidecomputer/omicron/pull/11286/changes#diff-7f7dc4fc81c6ac8eb1ef68ca0aa646fbb394bd16edc460cfbe6bfebb802968d7L441-R413

[sion42x]

Succeeded on Madrid + voxel. On Madrid:

[madrid]root@oxz_switch0:~# pilot host exec -c 'svcs -H -o state oxide/sled-agent; grep -c "kind = .production" /opt/oxide/sled-agent/pkg/config.toml' 14-17
14  BRM42220081        ok: online
1
15  BRM42220046        ok: online
1
16  BRM42220007        ok: online
1
17  BRM42220004        ok: online
1

[madrid]root@oxz_switch0:~# pilot host exec -c 'zoneadm list | grep -c oxz_switch; svcs -H -o state pumpkind 2>/dev/null' 14-17
14  BRM42220081        ok: 1
online
15  BRM42220046        ok: 0
online
16  BRM42220007        ok: 1
online
17  BRM42220004        ok: 0
online

[madrid]root@oxz_switch0:~# svcs -H -o state dendrite; svcprop -p config/mgmt dendrite 2>/dev/null; ls -l /dev/tofino/1
online
crw-------   1 root     sys      143,  1 Dec 28  1986 /dev/tofino/1

[madrid]root@oxz_switch0:~# svcprop -p config/board_rev dendrite
b

[madrid]root@oxz_switch0:~# pilot host exec -c 'grep -h -E "Found tofino node|Updated tofino|tofino device found" \
> /var/svc/log/oxide-sled-agent:default.log /var/svc/log/oxide-tofino-monitor:default.log \
> 2>/dev/null | cut -c1-150 | head -3' 14-17
14  BRM42220081        ok: {"msg":"Updated tofino","v":0,"name":"SledAgent","level":30,"time":"1986-12-28T00:00:45.226661561Z","hostname":"BRM42220081","pid":674,"component":"Ha
Dec 28 00:00:45.282 INFO tofino device found, unit: tofino-monitor
15  BRM42220046        ok: 
16  BRM42220007        ok: {"msg":"Updated tofino","v":0,"name":"SledAgent","level":30,"time":"1986-12-28T00:00:45.158379905Z","hostname":"BRM42220007","pid":674,"component":"Ha
Dec 28 00:00:45.208 INFO tofino device found, unit: tofino-monitor
17  BRM42220004        ok: 
[madrid]root@oxz_switch0:~# 

[sion42x]

On voxel:

root@helios1:~/oxide/omicron# for i in g0 g1 g2; do echo "== $i"; voxel host exec -c '
svcs -H -o state oxide/sled-agent
grep -c "kind = .virtual" /opt/oxide/sled-agent/pkg/config.toml
grep "voxel role" /opt/cargo-bay/sled-config.toml
grep -h -E "found SoftNPU 9p device|probe failed|busy|skipping pumpkind|SledModeConfig|unknown field" \
  /var/svc/log/oxide-sled-agent:default.log | cut -c1-140
zoneadm list | grep -c oxz_switch' $i 2>&1 | grep -v "INFO set propolis"; done
== g0
online
2
# voxel role: scrimlet
{"msg":"not an Oxide sled; skipping pumpkind","v":0,"name":"SledAgent","level":30,"time":"2026-09-12T18:19:13.579336523Z","hostname":"g0","p
{"msg":"found SoftNPU 9p device","v":0,"name":"SledAgent","level":30,"time":"2026-09-12T18:19:14.575353458Z","hostname":"g0","pid":776,"comp
1
== g1
online
2
# voxel role: gimlet
{"msg":"not an Oxide sled; skipping pumpkind","v":0,"name":"SledAgent","level":30,"time":"2026-09-12T18:19:15.771940262Z","hostname":"g1","p
0
== g2
online
2
# voxel role: scrimlet
{"msg":"not an Oxide sled; skipping pumpkind","v":0,"name":"SledAgent","level":30,"time":"2026-09-12T18:19:19.654551391Z","hostname":"g2","p
{"msg":"found SoftNPU 9p device","v":0,"name":"SledAgent","level":30,"time":"2026-09-12T18:19:20.374239364Z","hostname":"g2","pid":789,"comp
1