[codex] Improve unifiedlog table tests

[grahamgilbert]

Summary

• add unifiedlog column, generator guard, and constraint coverage
• verify full row mapping for log show JSON output
• cover JSON error handling
• fix time_zone_name output key to match the declared column

Validation

• GOCACHE=/tmp/osquery-extension-go-cache go test ./tables/unifiedlog
• GOCACHE=/tmp/osquery-extension-go-cache go test ./...