[Platform][OpenAI] Add file input normalizer

[glengemann]

Q	A
Bug fix?	no
New feature?	yes
Docs?	no
Issues	Fix #...
License	MIT

Hi there!

Currently, if you try to add a file into the message input, the following error is thrown:

// PHP Fatal error:  Uncaught Symfony\Component\Serializer\Exception\NotNormalizableValueException: Could not normalize object of type "Symfony\AI\Platform\Message\Content\File", no supporting normalizer found.

Message::ofUser(
    'What is this document about?',
    File::fromFile(dirname(__DIR__, 2).'/fixtures/document.pdf'),
)

This PR adds support for including files as inputs in the OpenAI completions API, as described in the official documentation. The implementation follows the same approach already used for Gemini and Anthropic.

[chr-hertel]

One thing that got me thinking is that we use the OpenAI contract mostly as default - meaning that the normalizers are mostly in namespace Symfony\AI\Platform\Contract\Normalizer - but I tested with Ollama and Cerebras, that are basically using that contract, but got errors - so i guess it's fine to only have it for OpenAI instead of default.

[chr-hertel]

Thanks for this, would love to merge that - one tiny thing noted about the normalizer name.

[chr-hertel]

Thank you @glengemann.