fix(core): clear consumed stdio read buffer

packages/core-internal/src/shared/stdio.ts

@@ -31,7 +31,8 @@ export class ReadBuffer {
             }
 
             const line = this._buffer.toString('utf8', 0, index).replace(/\r$/, '');
-            this._buffer = this._buffer.subarray(index + 1);
+            const remainder = this._buffer.subarray(index + 1);
+            this._buffer = remainder.length === 0 ? undefined : remainder;
 
             try {
                 return deserializeMessage(line);

packages/core-internal/test/shared/stdio.test.ts

@@ -22,6 +22,18 @@ test('should only yield a message after a newline', () => {
     expect(readBuffer.readMessage()).toBeNull();
 });
 
+test('should clear consumed buffer when newline is the final byte', () => {
+    const readBuffer = new ReadBuffer();
+
+    readBuffer.append(Buffer.from(JSON.stringify(testMessage) + '\n'));
+    expect(readBuffer.readMessage()).toEqual(testMessage);
+    expect((readBuffer as unknown as { _buffer?: Buffer })._buffer).toBeUndefined();
+
+    const nextChunk = Buffer.from(JSON.stringify(testMessage));
+    readBuffer.append(nextChunk);
+    expect((readBuffer as unknown as { _buffer?: Buffer })._buffer).toBe(nextChunk);
+});
+
 test('should be reusable after clearing', () => {
     const readBuffer = new ReadBuffer();