Tell a refused PDU apart from a session that failed (#82)

* Regression tests for telling a refused PDU from a failed session

* Export PduRefusedError so a refusal can be told from a session failure

* Document telling a refused PDU from a session failure

* Share the malformed-PDU fixtures and pin that a dead stream is no refusal

* Say once what a refusal is, and why the header type is published

* Fold the refused-response test into the one that already staged it

* Drop the refusal-rate claim the interop finding does not support

* Build a raw PDU one way, sequence number included

* Follow PduRefusedError into pdu-refusal.ts

test/raw-pdus.ts

@@ -0,0 +1,40 @@
+import assert from 'node:assert/strict';
+import type { PduObjectInput } from '../src/pdu.ts';
+import { objToPdu } from '../src/pdu.ts';
+
+/** The octets a test writes straight to a socket, which objToPdu builds for every valid PDU. */
+export function pduBytes(input: PduObjectInput): Buffer {
+	const { buffer } = objToPdu(input);
+
+	assert.ok(buffer);
+
+	return buffer;
+}
+
+/** The same, wearing a command id the command table defines nothing for. */
+export function withUnknownCmdId(input: PduObjectInput): Buffer {
+	const buffer = pduBytes(input);
+
+	buffer.writeUInt32BE(0x00010001, 4);
+
+	return buffer;
+}
+
+/** command_length honoured, so the stream stays in sync, with the declared body cut short. */
+export function shortened(input: PduObjectInput, octets: number): Buffer {
+	const buffer = pduBytes(input);
+	const cut = buffer.subarray(0, buffer.length - octets);
+
+	cut.writeUInt32BE(cut.length, 0);
+
+	return cut;
+}
+
+/** The same, with a message_state TLV declaring four octets of value and carrying one. */
+export function truncatedTlv(input: PduObjectInput): Buffer {
+	const appended = Buffer.concat([pduBytes(input), Buffer.from('0427000401', 'hex')]);
+
+	appended.writeUInt32BE(appended.length, 0);
+
+	return appended;
+}