persistence: replace try! with guard-let in defaultMemory

[ArasanDev]

Type of Change

• Bug fix

Motivation and Context

In MachineConfig.defaultMemory, the MemorySize initializer is called with try! inside a computed property getter:

return try! MemorySize("\(gb)gb")

This is a NeverForceTry violation — the exemption for static let initializers does not apply to computed property getters. The input is a runtime-derived string ("\(gb)gb"), so a defensive fallback is appropriate.

Replaced with guard let + fatalError:

guard let size = try? MemorySize("\(gb)gb") else {
    fatalError("invalid default memory size: \(gb)gb")
}
return size

Testing

• Tested locally
• make test passes