embedded image_export_mode should include image bytes in exported document #132

Description:

When requesting image_export_mode: "embedded" the exported document's pictures[] entries contain no image data (image field null). Expected behavior is that embedded mode produces the actual embedded image bytes/data in the pictures section of the exported document (same semantics as convert endpoint when embedded is requested).

How to reproduce:

POST a document to /v1/convert/file/async (or /v1/chunk/.../file/async if chunking endpoint supports convert options).
Set convert_options.image_export_mode=embedded (or request JSON with image_export_mode: "embedded").
Inspect returned document.pictures[] entries — image remains null.

Actual behavior:

pictures[].image is null for embedded mode.

Expected behavior:

pictures[].image contains the embedded image content (base64 or bytes as used by other export paths).