feat: Allow combining -p and stdin for prompt input (#4406)

Co-authored-by: Allen Hutchison <adh@google.com>

packages/cli/src/utils/readStdin.ts

@@ -5,14 +5,26 @@
  */
 
 export async function readStdin(): Promise<string> {
+  const MAX_STDIN_SIZE = 8 * 1024 * 1024; // 8MB
   return new Promise((resolve, reject) => {
     let data = '';
+    let totalSize = 0;
     process.stdin.setEncoding('utf8');
 
     const onReadable = () => {
       let chunk;
       while ((chunk = process.stdin.read()) !== null) {
+        if (totalSize + chunk.length > MAX_STDIN_SIZE) {
+          const remainingSize = MAX_STDIN_SIZE - totalSize;
+          data += chunk.slice(0, remainingSize);
+          console.warn(
+            `Warning: stdin input truncated to ${MAX_STDIN_SIZE} bytes.`,
+          );
+          process.stdin.destroy(); // Stop reading further
+          break;
+        }
         data += chunk;
+        totalSize += chunk.length;
       }
     };