Don't allow undecodable bytes to be accepted in ProcessProposal

[cmwaters]

Currently, if a tx as an array of bytes fails to be decoded, we skip over the tx but don't actually reject it:

celestia-app/app/process_proposal.go

Lines 61 to 66 in a692125

	if err != nil {
	// we don't reject the block here because it is not a block validity
	// rule that all transactions included in the block data are
	// decodable
	continue
	}

This effectively means that a proposer is able to include arbitrary bytes in the block and leverage data availability without incurring any cost. If we plan to have a minimal fee market in place for v2, then part of this requires adding a new block validity rule that rejects any tx that is not decodable

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[evan-forbes]

I think this is a good idea reguardless of implementing a mechanism to stop proposers from including 0 fee transactions

[evan-forbes]

iirc, the reasoning for keeping this, was that we didn't explicitly discuss adding this block validity rule and its block validity rule that is not easily fraud provable (while also bounding fraud proof size to something smaller than the block)

[rootulp]

Interesting, even though we don't already reject proposals that include a non-decodable tx. We have a test case for it. The test case rejects the proposal block here with the error:

"normal tx at index 3 can not be appended after blob tx"