fix: make yaml formatting side-effect free

[2830500285]

Summary

• make YAML formatting return bytes like the other output formats instead of writing directly to stdout
• keep ShowJSON / iterator callers responsible for writing output, so YAML can be buffered, counted, and paged consistently
• add regression coverage for the formatter side effect and the ShowJSON YAML output path

No linked issue; I found this while looking for another runtime correctness fix after the currently open issues were already covered by existing PRs.

Validation

• go test ./pkg/cmd -run 'TestFormatJSON|TestShowJSONYAML|TestShowJSONIterator'
• go test ./... -run 'TestFormatJSON|TestShowJSONYAML|TestShowJSONIterator'
• go build ./...

Note: I also attempted go test ./... locally, but this Windows environment is missing /bin/bash for the autocomplete tests and does not have the mock server running on localhost:4010 for generated command tests.