fix(certs): respect --json output in certs setup

[manasa-bhagwat]

Summary

Fixes fly certs setup ignoring --json output mode.

• Add JSON output handling in runCertificatesSetup in internal/command/certificates/root.go
• When --json is set, return API response via render.JSON(...) and skip human-readable DNS instructions
• Preserve existing non-JSON behavior unchanged

Fixes #4724.

Why

fly certs setup --json should be machine-readable for automation/IaC workflows.
Before this change, the command always printed human-oriented setup text, which broke JSON-driven scripting.

Test plan

• Build local binary:
  • go build -o .\bin\flyctl.exe .
• Manual verify JSON mode:
  • .\bin\flyctl.exe certs setup <hostname> --json -a <app>
  • Confirm output is JSON (no DNS Setup Options human text)
• Manual verify default mode:
  • .\bin\flyctl.exe certs setup <hostname> -a <app>
  • Confirm existing human-readable DNS setup output remains

Notes

• This PR is intentionally scoped to issue fly certs --json arg is ignored #4724 (certs setup).
• No behavior changes to other certs subcommands in this PR.