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/session-extras.test.ts

@@ -20,12 +20,13 @@ import { LinkGate } from '../src/link-gate.ts';
 import { Reassembler, decodeSegments } from '../src/reassembly.ts';
 import { Session } from '../src/session.ts';
 import { DlrMerger } from '../src/dlr-merger.ts';
+import { PduRefusedError } from '../src/pdu-refusal.ts';
+import { objToPdu } from '../src/pdu.ts';
 import { checkSessionOptions } from '../src/session-options.ts';
 import { client } from '../src/client.ts';
 import { closeAfter, closeListenerAfter } from './teardown.ts';
 import { consts } from '../src/defs/constants.ts';
 import { errors } from '../src/defs/errors.ts';
-import { objToPdu } from '../src/pdu.ts';
 import { paramNumber, paramText } from '../src/defs/types.ts';
 import { server } from '../src/server.ts';
 import { silentLog } from '../src/log.ts';
@@ -499,14 +500,16 @@ describe('reconnect', () => {
 		const events: string[] = [];
 
 		session.on('close', () => { events.push('close'); });
-		session.on('sessionError', () => { events.push('sessionError'); });
+		session.on('sessionError', err => {
+			events.push(err instanceof PduRefusedError ? 'refused' : 'sessionError');
+		});
 
 		const reconnected = once<true>(resolve => { session.on('reconnected', () => { resolve(true); }); });
 
 		peerOf(smpp).sock.write(unreadablePdu);
 		await reconnected;
 
-		assert.deepEqual(events, ['sessionError']);
+		assert.deepEqual(events, ['sessionError'], 'an unframeable stream is the link dying, not one PDU refused');
 	});
 
 	test('refuses a backoff that would retry without pausing', () => {