Function validate_back_populates 

pub fn validate_back_populates<Source: Model, Target: Model>(
    source_field: &str,
) -> Result<(), String>

Validate that back_populates is symmetric between two models.

If Hero::team has back_populates = "heroes", then Team::heroes must exist and have back_populates = "team".

Returns Ok(()) if valid, Err with message if invalid.