Fix checkpoint backward compatibility, improve metadata error reporting

[jlamypoirier]

✨ Description

• Fix a bug introduced in Checkpoint metadata #28 where older checkpoints were not loaded in the flat format.
• Re-raise errors in load_metadata as ValueError to prevent them from being hidden when called during validation.
• Add unknown fields to _to_dict so they are more visible.

🔍 Type of change

Select all that apply:

• 🐛 Bug fix (non-breaking change that addresses a specific issue)
• 🚀 New feature (non-breaking change that adds functionality)
• ⚠️ Breaking change (a change that could affect existing functionality)
• 📈 Performance improvement/optimization (improves speed, memory usage, or efficiency)
• 🛠️ Code refactor (non-functional changes that improve code readability, structure, etc.)
• 📦 Dependency bump (updates dependencies, including Dockerfile or package changes)
• 📝 Documentation change (updates documentation, including new content or typo fixes)
• 🔧 Infrastructure/Build change (affects build process, CI/CD, or dependencies)