ethgas: use the arbitrum gas price paid reader for robinhood chains

Adds robinhood (4663) and robinhood-testnet (46630) to
CustomGasPricePaidReaders. Both are Arbitrum Orbit chains running
Arbitrum Nitro, so they belong with the other arbitrum entries.

Without this they fall through to DefaultGasPricePaidReader, which
reports each transaction's GasFeeCap -- the maximum the sender was
willing to pay. On these chains that overstates the price actually
paid: eth_maxPriorityFeePerGas returns 0, and ordering is
first-come-first-served by sequencer arrival, so tips cannot affect
inclusion and effectively nothing is paid above the base fee.
arbitrumGasPricePaidReader reports block.BaseFee() instead, which is
what was really paid.

Affects gas statistics only, not transaction correctness.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: pkieltyka

ethgas/readers.go

@@ -18,6 +18,8 @@ var CustomGasPricePaidReaders = map[uint64]GasPriceReader{
 	421613: arbitrumGasPricePaidReader, // arbitrum goerli
 	421614: arbitrumGasPricePaidReader, // arbitrum sepolia
 	200:    arbitrumGasPricePaidReader, // arbitrum xdai
+	4663:   arbitrumGasPricePaidReader, // robinhood
+	46630:  arbitrumGasPricePaidReader, // robinhood testnet
 }
 
 func DefaultGasPriceBidReader(block *ethmonitor.Block) []*big.Int {