Function encode_big_int_caster 

pub fn encode_big_int_caster(value: &BigInt) -> Bytes

Encodes a BigInt using Go’s BigIntCaster format.

The wire format is sign-byte-plus-magnitude:

• nil is represented separately by callers as [0]
• zero is encoded as [0, 0]
• positive values are encoded as [0, ..magnitude_be]
• negative values are encoded as [1, ..magnitude_be]