ArgumentCountError in logHeaders when HTTP body contains percent signs

## Description

The `logHeaders()` method in `Transport.php` uses `sprintf()` with the HTTP message body, which causes an `ArgumentCountError` when the body contains `%` characters followed by format specifier characters (common in URL-encoded data).

## Steps to Reproduce

1. Perform a bulk indexing operation where document content contains `%` characters (e.g., URL-encoded strings like `%20`, `%3A`, etc.)
2. The error occurs during the `sprintf()` call in `logHeaders()`

## Error Message

```
ArgumentCountError in sprintf called at vendor/elastic/transport/src/Transport.php (280)
```

## Stack Trace

```
in Elastic\Transport\Transport::logHeaders called at Transport.php (296)
in Elastic\Transport\Transport::logRequest called at Transport.php (325)
in Elastic\Transport\Transport::sendRequest called at Client.php (202)
in Elastic\Elasticsearch\Client::sendRequest
in Elastic\Elasticsearch\Client::bulk
```

## Root Cause

In `src/Transport.php` lines 275-284:

```php
private function logHeaders(MessageInterface $message): void
{
$this->logger->debug(sprintf(
"Headers: %s\nBody: %s",
json_encode($message->getHeaders()),
(string) $message->getBody() // <-- If this contains %, sprintf fails
));

$message->getBody()->rewind();
}
```

The `sprintf()` arguments are evaluated **before** `$this->logger->debug()` is called, so even with a `NullLogger`, the error still occurs when the body contains patterns like `%s`, `%d`, `%20`, etc.

## Suggested Fix

Replace `sprintf()` with string concatenation:

```php
private function logHeaders(MessageInterface $message): void
{
$this->logger->debug(
"Headers: ".json_encode($message->getHeaders()).
"\nBody: ".(string) $message->getBody()
);

$message->getBody()->rewind();
}
```

## Environment

- PHP: 8.1+
- elastic/transport: v9.0.1
- elasticsearch/elasticsearch: ^9.1

Start in src/Transport.php at Transport::logHeaders and reproduce the bulk request with a body containing URL-encoded percent sequences such as %20. Done means logging completes without ArgumentCountError, the request body is still rewound, and the existing request flow continues normally.

Written by the indexing model from the issue text.