audit test coverage for spec compliance

[nothingmuch]

it appears not everything BIP 77 specifies was implemented correctly, see #843

now that the spec is merged, we should make a list of requirements to check for in the spec, add links to the relevant sections from doc comments in the corresponding tests where they exist, and open issues for spec requirements lacking coverage

[benalleng]

In adding some coverage in #928 spacebear found that our spec has perhaps an unclear directive on what to do in the case of a malicious modification of the pjos param in backwards-compatible receivers.

rust-payjoin/payjoin/src/core/receive/v2/mod.rs

Lines 398 to 422 in 0aa1509

	fn unchecked_from_payload(
	&mut self,
	payload: &str,
	) -> Result<v1::UncheckedProposal, ReplyableError> {
	let (base64, padded_query) = payload.split_once('\n').unwrap_or_default();
	let query = padded_query.trim_matches('\0');
	log::trace!("Received query: {query}, base64: {base64}"); // my guess is no \n so default is wrong
	let (psbt, mut params) =
	parse_payload(base64, query, SUPPORTED_VERSIONS).map_err(ReplyableError::Payload)?;
	// Output substitution must be disabled for V1 sessions in V2 contexts.
	//
	// V2 contexts depend on a payjoin directory to store and forward payjoin
	// proposals. Plaintext V1 proposals are vulnerable to output replacement
	// attacks by a malicious directory if output substitution is not disabled.
	// V2 proposals are authenticated and encrypted to prevent such attacks.
	//
	// see: https://github.com/bitcoin/bips/blob/master/bip-0078.mediawiki#unsecured-payjoin-server
	if params.v == Version::One {
	params.output_substitution = OutputSubstitution::Disabled;
	}
	let inner = v1::UncheckedProposal { psbt, params };
	Ok(inner)
	}

We have already had some discussion #928 (comment) about whether we should continue to modify the pjos param or error if we found that it has changed.