Cover both branches: single alert template (isSingle=true with 1 alert)
and grouped alert template (isSingle=false or multiple alerts). Tests
verify subject format, HTML structure, and text body generation.
Coverage improved from 66.7% to 100%.
Test coverage for titleCase, formatDuration, pluralize, formatMetricValue,
and formatMetricThreshold functions. 71 test cases covering edge cases,
boundary conditions, and all metric type variants.