Add length validations for error tracking errors and events

What does this MR do and why?

This MR adds missing model validations for errors and events. These validations mirror the actual database constraints and prevent exceptions like ActiveRecord::StatementInvalid: PG::CheckViolation and raise ActiveRecord::RecordInvalid instead.

Contributes to #345168 (closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.