RPC: Omit P2SH for asset scripts

[Tabakkui]

Problem

decodescript adds a p2sh field for every decoded output except an
already-typed scripthash. Asset scripts are not redeem scripts, so reporting
a P2SH wrapper for new_asset, transfer_asset, reissue_asset, and
restricted asset data is misleading.

Approach

Recognize all asset output types before adding the derived p2sh field. Keep
the existing behavior for ordinary script types, including the P2SH-derived
address for a normal P2PKH script.

Tests

• git diff --check
• python3 -m py_compile test/functional/rpc_decodescript.py
• Updated rpc_decodescript.py to assert that asset and owner scripts omit
  p2sh, while ordinary P2PKH scripts still include it.
• Full functional execution was not run locally because this Pi checkout has
  no configured/build ravend; CI/authorized PC build is the required gate.

Risk and exclusions

This changes only the presentation of the decodescript RPC result. It does
not change script classification, validation, consensus, address generation,
or asset serialization.

Fixes #388