Fix output buffering logic to properly capture echo and var_dump output

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

Author: Copilot

src/WP_CLI/Shell/REPL.php

@@ -57,6 +57,7 @@ public function start() {
 				if ( 0 < strlen( $out ) ) {
 					echo rtrim( $out, "\n" ) . "\n";
 				}
+				ob_start();
 				if ( ! $this->quiet ) {
 					echo '=> ';
 					var_dump( $evl );