refactor(types): restrict visibility of NsPayloadByteLen::from_usize

[CreeptoGengar]

Restrict the visibility of NsPayloadByteLen::from_usize method to crate-level since it's only used internally within the crate. This improves encapsulation and removes a TODO comment.

• Changed visibility from pub to pub(crate)
• Removed TODO comment about visibility restriction

[CLAassistant]

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