fix: add missing translations for image handling in MCP tools

Add missing thumbnail and image count translations for 15 languages:
- thumbnails.failedToLoad and thumbnails.altText in common.json
- execution.imageCountTooltip in mcp.json

This fixes the check-translations CI failure by providing proper
localized strings for the new image handling features.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: shivangag