Ensure that all used error tracking fixtures are valid

What does this MR do and why?

This is a follow-up of !78199 (merged) where a JSON schema typebug was fixed.

This MR ensures that all used spec fixtures in spec/fixtures/error_tracking/*event*.json are validated and actually valid to prevent future errors like !78199 (merged).

How to set up and validate locally

$ bin/rspec spec/services/error_tracking/collect_error_service_spec.rb  spec/lib/error_tracking/collector/payload_validator_spec.rb

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.