feat: more `Content` validation [APE-1546]

[antazoey]

What I did

while attempting to fix FastAPI stuff, I made content validate better in some edge cases (weird root models, None types, and direct-Paths)

How I did it

How to verify it

Checklist

• [ ] Passes all linting checks (pre-commit and CI jobs)
• [ ] New test cases have been added and are passing
• [ ] Documentation has been updated
• [ ] PR title follows Conventional Commit standard (will be automatically included in the changelog)