fix: correct length-coded number size for the 3-byte range (#4500)

* fix: correct length-coded number size for the 3-byte range

* test: align new test names with existing tag naming

Author: lazerg

lib/packets/packet.js

@@ -1135,7 +1135,7 @@ class Packet {
       return 3;
     }
     if (n < 0xffffff) {
-      return 5;
+      return 4;
     }
     return 9;
   }

test/unit/packets/test-length-coded-number.test.mts

@@ -39,3 +39,42 @@ describe('writeLengthCodedNumber / readLengthCodedNumber roundtrip', () => {
     strict.strictEqual(roundtrip(0x1ffffffffff), 2199023255551);
   });
 });
+
+describe('lengthCodedNumberLength matches the bytes writeLengthCodedNumber emits', () => {
+  const writtenBytes = (n: number): number => {
+    const buffer = Buffer.alloc(16);
+    const packet = new Packet(0, buffer, 0, buffer.length);
+    packet.offset = 0;
+    packet.writeLengthCodedNumber(n);
+    return packet.offset;
+  };
+
+  it('1-byte range', () => {
+    strict.strictEqual(Packet.lengthCodedNumberLength(250), writtenBytes(250));
+  });
+
+  it('0xFC tag', () => {
+    strict.strictEqual(
+      Packet.lengthCodedNumberLength(0xfffe),
+      writtenBytes(0xfffe)
+    );
+  });
+
+  it('0xFD tag', () => {
+    strict.strictEqual(
+      Packet.lengthCodedNumberLength(0xffff),
+      writtenBytes(0xffff)
+    );
+    strict.strictEqual(
+      Packet.lengthCodedNumberLength(0xfffffe),
+      writtenBytes(0xfffffe)
+    );
+  });
+
+  it('0xFE tag', () => {
+    strict.strictEqual(
+      Packet.lengthCodedNumberLength(0xffffff),
+      writtenBytes(0xffffff)
+    );
+  });
+});