feat: enabling routing for Supermicro GB300s

[krish-nvidia]

The Supermicro GB300 reports Vendor: "Supermicro", so it was getting sent to the supermicro.rs path. But it's actually an OpenBMC box running the same GB Redfish tree as GB200 which nvidia_gbx00 already handles. This sends it there instead.

Evidence (from the tray dump):

• Service root: Vendor: "Supermicro", Product: "GB NVL" but Managers/BMC_0 is "OpenBmc", not an Supermicro BMC.
• No Oem/Supermicro anywhere
• Same GB200 shape: System_0 + HGX_Baseboard_0, standard Bios/Settings, etc

Changes:

• service_root.rs: Supermicro + GB NVL → NvidiaGBx00
• nvidia_gbx00.rs: add a vendor-agnostic is_gb300() helper. Groundwork for the GB300-specific tweaks, not wired up yet.
  • Both Supermicro and Lenovo present GB300 1CPU:2GPU Board PC in the HGX_Baseboard_0 system model