Issue #3561693: Fix InvalidArgumentException in saveFormValues() for image fields

AutomatorBaseAction::saveFormValues() calls $item->get('value') on all field items, but image fields do not have a 'value' property (they use target_id, alt, title, width, height). This causes an InvalidArgumentException: "Property value is unknown."

Add a property existence check before accessing field item properties. If the field type does not define the expected property (e.g. image fields with formElementProperty='value'), return early since the automator (e.g. LlmRewriteImageFilename) stores data directly on the referenced File entity rather than on the field item.

This is not Canvas-specific — core ImageItem also lacks 'value'.

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

Closes #3561693