Fix ProUpServTx and ProRegTx serialization/deserialization

[LexxXell]

• Fixed ProUpServTx parsing for v1 (payloadSig as varbytes) and v2+ (raw 90/96 bytes).
• Added support for optional platform fields in ProUpServTx (mn_type == 1).
• Ensured correct cursor alignment to satisfy extraPayload size assertion.
• Fixed ProRegTx payloadSig handling to consistently use varbytes (including empty signatures).
• Removed incorrect fixed-length assumptions for ProRegTx signatures.

All related serialization tests now pass.