xpc: replace precondition+force-unwrap with guard-let in XPCMessage

[ArasanDev]

Type of Change

• Bug fix

Motivation and Context

In XPCMessage, two methods use a precondition(x != nil) + immediate force-unwrap pattern, which violates NeverForceUnwrap:

// error()
let item = try? JSONDecoder().decode(ContainerXPCError.self, from: data)
precondition(item != nil, "expected to receive a ContainerXPCXPCError")
throw ContainerizationError(item!.code, message: item!.message)

// set(error:)
let data = try? JSONEncoder().encode(serializableError)
precondition(data != nil)
set(key: Self.errorKey, value: data!)

Both replaced with guard let + preconditionFailure, which expresses the same invariant without force-unwrap. Also fixed a typo in the error message (ContainerXPCXPCError → ContainerXPCError).

Testing

• Tested locally
• make test passes