Add tests for k8s instance, log, and metric modules

[fedemaleh]

Summary

• Add BATS unit tests for k8s/instance (7 + 10 tests), k8s/log (6 tests), and k8s/metric (8 + 1 + 35 tests)
• Remove debugging logs from log/build_context, log/log, and metric/build_context — stdout must be JSON-only in these modules
• Use log error from k8s/logging for error blocks (❌/💡/🔧) instead of raw echo >&2
• Add load logging workflow step with function output in log.yaml and metric.yaml

Test plan

• bats k8s/instance/tests/ — 17 tests pass
• bats k8s/log/tests/ — 6 tests pass
• bats k8s/metric/tests/ — 44 tests pass