HostPhase2Status could use an enum when boot disk is unknown

To decide whether a sled is missing from inventory, we use the following method:

impl SledAgentUpdateStatus {
    /// Update status suitable for a given sled ID that isn't present in
    /// inventory.
    pub fn unknown(sled_id: SledUuid) -> Self {
        Self {
            sled_id,
            zones: IdOrdMap::new(),
            host_phase_2: HostPhase2Status {
                boot_disk: Err("unknown".to_string()),
                slot_a_version: TufRepoVersion::Unknown,
                slot_b_version: TufRepoVersion::Unknown,
            },
        }
    }
}

How we check the boot disk is a bit fragile due to string equality. It would be nice to return an enum variant.

Found in https://github.com/oxidecomputer/omicron/pull/10476#discussion_r3306051658

Start with SledAgentUpdateStatus::unknown and the HostPhase2Status boot_disk field shown in the issue. Search for checks that compare the boot-disk error to the "unknown" string and trace their callers. Done means the unknown state is represented by an enum variant rather than fragile string equality, with affected behavior preserved.