fix: Use standard error fields for rate limit logs

Description

Updates rate limit error logging to use Labkit’s standard structured error fields.

Previously, Labkit::RateLimit::Evaluator logged Redis/rate limit failures with a custom error field and a message: "rate_limit_error" value. This MR changes those log entries to use the shared field constants:

• error_type for the rate limit error category
• class_name for the exception class
• error_message for the exception message

This makes rate limit error logs consistent with the rest of Labkit’s structured logging conventions and easier to query with standard error fields.

Related Issues

• Spawned from this thread gitlab-org/gitlab!239802 (comment 3449841605)