Fail closed when CSPRNG is unavailable during nonce generation

Replace the weak nonce fallback in create_login_nonce() with a
hard failure. The fallback used wp_hash() with predictable inputs
(user_id, wp_rand, microtime) and was defensive code from the
PHP 5.x era. On PHP 7+ (the plugin minimum is 7.2), random_bytes()
uses OS-level CSPRNG sources that do not fail under normal conditions.

If the CSPRNG is broken, generating a weak nonce is worse than
refusing to proceed. Both callers already handle the false return
with wp_die().

Closes #860

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dknauss

class-two-factor-core.php

@@ -1208,7 +1208,7 @@ public static function create_login_nonce( $user_id ) {
 		try {
 			$login_nonce['key'] = bin2hex( random_bytes( 32 ) );
 		} catch ( Exception $ex ) {
-			$login_nonce['key'] = wp_hash( $user_id . wp_rand() . microtime(), 'nonce' );
+			return false;
 		}
 
 		// Store the nonce hashed to avoid leaking it via database access.