I'm encountering a PHP Fatal error: Uncaught Error: Class "Google\Cloud\Dialogflow\V2\SessionsClient" not found in my WordPress child theme's functions.php file. I'm trying to integrate with the Google Cloud Dialogflow API using the official PHP client library.

Here's a summary of my setup:

• WordPress Installation: Running on a standard hosting environment.
• Theme: Using the Salient theme with a custom child theme.
• Dialogflow PHP Client: Installed using Composer within my child theme directory (/wp-content/themes/salient-child/). I have verified that the vendor directory and the google/cloud-dialogflow library are present.
• Composer Autoloader: The line require_once __DIR__ . '/vendor/autoload.php'; is included at the very top of my child theme's functions.php file (as the first executable line after <?php).
• Error Location: The error occurs within a function that attempts to instantiate the Google\Cloud\Dialogflow\V2\SessionsClient class.

Here's the relevant snippet from my functions.php:

    <?php
    
    require_once __DIR__ . '/vendor/autoload.php';
    
    // ... other functions ...
    
    use Google\Cloud\Dialogflow\V2\SessionsClient;
    use Google\Cloud\Dialogflow\V2\TextInput;
    use Google\Cloud\Dialogflow\V2\QueryInput;
    
    function hvac_chatbot_get_dialogflow_response($user_message) {
        $credentials_path = '/path/to/my/credentials.json'; // Actual path is set
        $project_id = 'your-project-id'; // Actual ID is set
        // ... other code ...
        try {
            $sessionsClient = new SessionsClient(['credentials' => $credentials_path]);
            // ... rest of the code ...
        } catch (\Exception $e) {
            error_log('Dialogflow API Error: ' . $e->getMessage());
            return 'Sorry, I encountered an error. Please try again later.';
        } finally {
            // ...
        }
    }
    
    // ... AJAX handler that calls the above function ...

What I've tried so far:

• Verified that the vendor directory and the Dialogflow library exist in the child theme.
• Confirmed that the require_once line for the autoloader is present and correctly placed in functions.php.

My Question:

Despite the autoloader being included and the library files being present, I am still getting the "Class not found" error. What could be preventing the Google\Cloud\Dialogflow\V2\SessionsClient class from being loaded? Are there any common pitfalls in WordPress or server configurations that might interfere with Composer's autoloader in a child theme? Any suggestions on further debugging steps would be greatly appreciated.