[Maintenance][DX] Add support for PDF generation abstraction

[NoResponseMate]

Summary

This PR adds support for sylius/pdf-generation-bundle as a modern alternative to the legacy PDF generation setup based on KnpSnappyBundle with a hardcoded wkhtmltopdf binary.

The new integration is opt-in and fully backward compatible — existing setups continue to work without any changes.

Motivation

The current PDF generation in RefundPlugin is tightly coupled to KnpSnappyBundle and wkhtmltopdf. The sylius/pdf-generation-bundle provides a proper abstraction layer with support for multiple adapters, context-based configuration, and a unified filesystem management — making it easier to swap PDF engines or customize the generation pipeline.

How to use it

To switch from the legacy PDF generation to the new bundle, set the legacy option to false in your configuration:

sylius_refund:
    pdf_generator:
        legacy: false

That's it. The plugin automatically configures a sylius_refund context in sylius/pdf-generation-bundle with the knp_snappy adapter and gaufrette storage, making it a drop-in replacement.

For more details on available adapters, storage options, and custom option processors, refer to the sylius/pdf-generation-bundle documentation.

Deprecations

The following classes and interfaces are deprecated and will be removed in 3.0:

Deprecated	Replacement
TwigToPdfGeneratorInterface	TwigToPdfRendererInterface from sylius/pdf-generation-bundle
TwigToPdfGenerator	TwigToPdfRendererInterface from sylius/pdf-generation-bundle
PdfOptionsGeneratorInterface	Option processors from sylius/pdf-generation-bundle
PdfOptionsGenerator	Option processors from sylius/pdf-generation-bundle
CreditMemoFileManagerInterface	PdfFileManagerInterface from sylius/pdf-generation-bundle
CreditMemoFileManager	PdfFileManager from sylius/pdf-generation-bundle

The corresponding services (sylius_refund.generator.twig_to_pdf, sylius_refund.generator.pdf_options, sylius_refund.manager.credit_memo_file) are also deprecated.

Issues

Fixes #329, fixes #480