FIX: apply font_color to PDFs PDFConverter generates

[Utkarsh Bahuguna (u7k4rs6)]

Fixes #3105.

Went with the option that doesn't change anything for people who don't pass a color:

• font_color now defaults to None.
• Generated PDFs set the fill color to font_color, or black when it isn't set (same as today), and set it again after each page break.
• Injection items without their own color still fall back to white when font_color isn't set, so hidden injected text keeps working. If font_color is set, they use it.
• font_color is in the identifier when set. Unset is dropped, so default PDFConverter hashes don't move. I left row_height/column_width/injection_items out to keep this small.

Tests go through convert_async and read the fill-color ops back out of the PDF content stream:

• generated PDF over several pages with no color, red and white
• injection with every combo of converter color and item color
• identifier with and without a color
• invalid colors still raise

The new ones fail on main. tests/unit/converter, registry and backend/test_converter_service: 2928 passed.