req-resp: Fix panic on connection closed for substream open failure

[lexnv]

When a new connection is reported to the request-response protocol, the protocol tracks the state of the connection if and only if it could open a substream with the remote peer.

We do not track the state of the connection in cases where the substream cannot be opened because the connection is closed.

A further ConnectionClosed event triggered a debug_assert assumption for this case. The assumption is wrong since opening substreams can fail which informs us that the connection is closed.

Connection details not tracked:

litep2p/src/protocol/request_response/mod.rs

Lines 267 to 284 in eed0f75

	Err(error) => {
	tracing::warn!(
	target: LOG_TARGET,
	?peer,
	protocol = %self.protocol,
	request_id = ?context.request_id,
	?error,
	"failed to open substream",
	);
	return self
	.report_request_failure(
	peer,
	context.request_id,
	RequestResponseError::Rejected(error.into()),
	)
	.await;
	}

Which later leads to panics:

litep2p/src/protocol/request_response/mod.rs

Lines 295 to 301 in d6fae55

	let Some(context) = self.peers.remove(&peer) else {
	tracing::error!(
	target: LOG_TARGET,
	?peer,
	"state mismatch: peer doesn't exist",
	);
	debug_assert!(false);

Related to: #290