Function validate_descriptor 

pub fn validate_descriptor(
    desc: &MsgDescHot,
    limits: &DescriptorLimits,
) -> Result<(), ValidationError>

Validate a descriptor against the given limits.

This performs transport-agnostic validation:

• Bounds checking for slots (if slot_count > 0)
• Payload length limits
• Inline payload constraints

Transport-specific validation (e.g., generation checks) happens in the transport.