fix(multimodal): Implement PDF path tracking in interactions

Problem:

• PDF uploads were processed but file paths not saved to database
• History API returned null for pdfPath even when PDFs were uploaded
• Inconsistency with image/audio path tracking

Solution:

• Add DocumentProcessingService.processMultipartFileWithPath()
• Store PDF paths in currentProcessingResults during processing
• Ensure PDF paths are persisted to interaction_devoir.pdf_path

Changes:

• DocumentProcessingService: New method returning ProcessingResultWithPaths
• MultimodalChatServiceImpl: Store PDF path in processing flow
• Maintains consistency with existing media file tracking

Closes IA-439