feat(broadcasts): add duplicate endpoint

[gabrielmfern]

This ports the Node method resend.broadcasts.duplicate(id) to Ruby for POST /broadcasts/{id}/duplicate, the endpoint added in resend/resend-openapi#115. The release carries 1.16.0 in its own commit. No examples/ change, following the convention that feature PRs here ship lib + spec only; the @note on the method is kept on purpose to explain what the copy is, since the response alone does not say.

Claude ran the new method against staging with RESEND_BASE_URL pointed at api.resend-staging.com, duplicating broadcast e6bd3843-3cc7-4c0d-b7fd-e01cbb514a16, then deleted the copy.

dup.rb

require "resend"

Resend.api_key = ENV["RESEND_API_KEY"]

result = Resend::Broadcasts.duplicate("e6bd3843-3cc7-4c0d-b7fd-e01cbb514a16")
puts "object: #{result[:object]}"
puts "id: #{result[:id]}"

$ RESEND_API_KEY=$(fish -c ds) RESEND_BASE_URL=https://api.resend-staging.com/ bundle exec ruby -I lib /tmp/dup.rb
object: broadcast
id: 7a102835-36a7-4886-a4ca-4970fd99ccc0

$ http DELETE https://api.resend-staging.com/broadcasts/7a102835-36a7-4886-a4ca-4970fd99ccc0 "Authorization: Bearer $(fish -c ds)"
HTTP/1.1 200 OK
{"object":"broadcast","id":"7a102835-36a7-4886-a4ca-4970fd99ccc0","deleted":true}

🤖 Generated with Claude Code

---

Summary by cubic

Adds the duplicate method to Resend::Broadcasts to call POST /broadcasts/{id}/duplicate, porting the existing Node SDK feature to Ruby. Bumps the gem version to 1.16.0 and adds a spec covering the new call. Implements the duplicate broadcast endpoint for the Ruby SDK as part of DEV-2094.

^{Written for commit 3eb86d5. Summary will update on new commits.}

[cubic-dev-ai]

cubic analysis

All reported issues were addressed across 4 files

Linked issue analysis

Linked issue: DEV-2094: Ruby SDK (internal, squad-devtools)

Status	Acceptance criteria	Notes
✅	Implement the Ruby broadcasts duplicate endpoint at POST /broadcasts/{id}/duplicate	The new Resend::Broadcasts.duplicate method sends a POST request to the expected endpoint, with a spec verifying the path, verb, and response.
✅	Release the Ruby SDK with the new endpoint	The gem version is bumped from 1.15.0 to 1.16.0 in the version file and lockfile.

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