feat: add measures and error rates for ads_client_operations_and_errors_hourly

[Almaju]

Summary

Stacked on top of #1430 (depends on that PR for the view definition). Adds measures and error rates to the ads_client_operations_and_errors_hourly view:

• Sum measures for every raw count column (operations, client errors, build cache errors, deserialization errors, HTTP cache outcomes)
• Category totals: build_cache_errors_total, deserialization_errors_total
• Error rate measures (formatted as percentages):
  • request_ads_error_rate, record_click_error_rate, record_impression_error_rate, report_ad_error_rate — each client error vs its matching operation
  • build_cache_error_rate — total build-cache errors vs op_new
  • deserialization_error_rate — total deserialization errors vs op_request_ads

No HTTP cache hit rate since a single cache request can have multiple outcomes, so summing them isn't a meaningful denominator.

Test plan

• CI passes