phsl / new-api

Fix TypeError when processing multimodal messages containing both text and images.
The error "textContent.text.trim is not a function" occurred when textContent
was null or textContent.text was not a string type.

Changes:
- Add comprehensive type checking for textContent.text access
- Implement getTextContent() utility function for unified content extraction
- Enhance error handling to support multimodal content display
- Fix message copy functionality to handle array-format content
- Improve message reset functionality to extract text content for retry
- Add user-friendly warnings when copying messages without text content

Technical improvements:
- Validate textContent existence and text property type before calling trim()
- Extract text content from multimodal messages for operations like copy/retry
- Maintain backward compatibility with string-format content
- Preserve all existing functionality while adding robust error handling

Fixes issues with:
- Image + text message processing
- Message copying from multimodal content
- Message retry with image attachments
- Error display for complex message formats

This ensures the playground component handles multimodal content gracefully
without breaking existing text-only message functionality.