Function validate_request 

pub fn validate_request(
    payload: &Value,
    max_body_size: usize,
    max_keys_per_request: usize,
) -> Result<(), ValidationError>

Run all validation checks on an incoming request payload.

This is the single entry point called by every process_* function in handlers.rs. It checks everything in one pass:

1. Payload size (10 MB limit)
2. JSON nesting depth (32 levels max)
3. Collection name validity
4. Key name validity (single key, batch keys, data map keys)
5. Field name validity (projections, joins, WHERE clause)

Returns Ok(()) if all checks pass, or the first ValidationError found.