Specify the antehandler

[evan-forbes]

Given its importance and slightly weird usage, we should add a spec for our antehandler. We should be sure to include what occurs to failed transactions and how it is used in prepare/processProposal.

[evan-forbes]

specifically this comment (copied for convenience)

good question! I don't think the ante handlers are really specified sufficiently anywhere, especially our usage of them, so I added #2044

that might involve updating the above! the reasoning behind saying "all remaining transactions types do not have to be valid" were these lines

celestia-app/app/process_proposal.go

Lines 55 to 61 in 65046dd

	sdkTx, err := app.txConfig.TxDecoder()(tx)
	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
	}

in the past, the rules were that validators could include invalid transactions in the block. It's very reasonable to see this as incorrect now since the most recent fix to use the entire antehandler instead of only incrementing the nonce!

celestia-app/app/process_proposal.go

Lines 72 to 79 in 65046dd

	// we need to increment the sequence for every transaction so that
	// the signature check below is accurate. this error only gets hit
	// if the account in question doens't exist.
	sdkCtx, err = handler(sdkCtx, sdkTx, false)
	if err != nil {
	logInvalidPropBlockError(app.Logger(), req.Header, "failure to incrememnt sequence", err)
	return reject()
	}

we might want to say something along the lines of "random tx data is permitted, but decodable sdk.Txs must pass all antehandler checks in order for the block to be valid".