[Bug]: Disallow coinbase transactions as donations or deposits

[djordon]

Bug - Disallow coinbase transactions as donations or deposits

1. Description

Right now, we allow any bitcoin transaction to serve as a donation or a deposit. Unfortunately, coinbase transactions screw everything up. We cannot spend them immediately, and the getrawtransaction RPC on bitcoin core does not give us all of the expected fields when the verbosity level is set to 2. So we should just disallow them as deposits and donations.

1.1 Context & Purpose

If a coinbase transaction is spent to the signers, we will encounter an error while processing the block. This could be problematic if there are sweep transactions confirmed within the block, as it would prevent us from correctly processing all transactions in the block. Although it is highly unlikely for a coinbase transaction to be spent to the signers, such an occurrence could block the signers from sweeping deposits and withdrawals until the issue is resolved.

2. Technical Details:

When processing transactions within a bitcoin block or a deposit request, check to see if each potential donation and deposit transaction is a coinbase transaction. For the deposit code path, check around here:

sbtc/signer/src/block_observer.rs

Lines 78 to 88 in 3853ad1

	// Fetch the transaction from either a block or from the mempool
	let Some(response) = client.get_tx(&self.outpoint.txid).await? else {
	return Ok(None);
	};
	// If the transaction has not been confirmed yet, then the block
	// hash will be None. The trasnaction has not failed validation,
	// let's try again when it gets confirmed.
	let Some(block_hash) = response.block_hash else {
	return Ok(None);
	};

log it and return an error if it's a coinbase transaction.

For the donations code path log it and continue around here:

sbtc/signer/src/block_observer.rs

Lines 454 to 455 in 3853ad1

	for tx in txs {
	tracing::trace!(txid = %tx.compute_txid(), "attempting to extract sbtc transaction");

These changes should effectively prevent signing deposits from coinbase transactions since we will never have a record of the deposit request in our database.

2.1 Acceptance Criteria:

• We do not accept coinbase transactions when processing a bitcoin block.

3. Related Issues and Pull Requests (optional):

Doing this with #1194 will help shore up processing of a bitcoin block.