feat(df): add rubygems wrapper commands behind the firewall proxy

Michael Eddingtonrequested to merge
df-mr-09-gem-commands into main

Adds the RubyGems-family wrapper subcommands, making gem and bundle usable end-to-end behind the firewall proxy.

Each is a thin dfcmd.NewProxyCmd wrapper: it forwards all arguments verbatim to the underlying binary while routing traffic through the local Dependency Firewall inspection proxy, checking every package download and upload against the project's policy.

Stacked on !3838 (merged) (the pypi slice), which is itself stacked on !3837 (merged) (the dfcmd helper) and carries the !3818 (merged) pm layer until those merge. Review/merge the stack in order. The net-new here is the two internal/commands/df/{gem,bundle} wrappers and their registration.

Testing

  • go build ./...
  • GITLAB_CI=true go test ./internal/commands/df/...
  • gofmt and golangci-lint clean.
  • make gen-docs regenerated the gem/bundle doc pages.

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