fix(genkit_openai): send non-image media as OpenAI file content parts

[irangarcia]

The content-part converter mapped every MediaPart to an image_url part, so PDFs and other non-image documents were rejected by the OpenAI API with "Invalid MIME type. Only image types are supported."

Branch non-image data URIs to ContentPart.fileData (the OpenAI file content part). OpenAI infers the document type from the filename extension, which Genkit's Media does not carry, so synthesize one from the MIME type. Image and remote-URL handling are unchanged.

[gemini-code-assist]

Code Review

This pull request updates the Genkit OpenAI converter to route non-image media (such as PDFs) as file content parts instead of image_url parts, synthesizing a filename based on the MIME type to prevent API rejection. Feedback suggests cleaning the MIME type string to strip parameters (e.g., ; charset=utf-8) before matching to avoid parsing failures and malformed file extensions.

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