This PR adds support for [Albert
API](https://github.com/etalab-ia/albert-api), the French government's
sovereign AI gateway. Albert provides an OpenAI-compatible API interface
for various language models while ensuring data sovereignty and
compliance with French/EU regulations.

- 🇫🇷 **Sovereign AI**: Albert is hosted and operated by the French
government
- 🔧 **OpenAI-compatible**: Uses the same API structure as OpenAI
- 🔒 **Data Privacy**: Ensures data remains within French/EU jurisdiction
- 🎯 **Public Sector Focus**: Designed for use by French public
administration

- Created Albert-specific model clients (GPTModelClient and
EmbeddingsModelClient) with configurable base URL
- Added PlatformFactory for easy Albert API initialization
- Reuses OpenAI's response converters for compatibility
- Supports both chat completions and embeddings endpoints

```php
use PhpLlm\LlmChain\Platform\Bridge\Albert\PlatformFactory;
use PhpLlm\LlmChain\Platform\Bridge\OpenAI\GPT;

$platform = PlatformFactory::create(
    apiKey: $_ENV['ALBERT_API_KEY'],
    albertUrl: $_ENV['ALBERT_API_URL']
);

$model = new GPT('gpt-4o');
$chain = new Chain($platform, $model);
```

- Added unit tests for PlatformFactory
- Added example script demonstrating Albert API usage with RAG context
- All tests pass and code follows project standards

- Updated README to list Albert as a supported platform
- Added example file showing how to use Albert API
- Included context about French AI strategy in the example

This implementation allows French public sector organizations to
leverage LLM Chain while maintaining data sovereignty and regulatory
compliance.

Closes #346