#216 feat: add Broadcasts.cancel method

Summary

• Adds Resend::Broadcasts.cancel(broadcast_id), calling POST /broadcasts/:id/cancel to cancel a queued or scheduled broadcast.
• Mirrors Emails.cancel's shape (no params, id-only).

Part of the broadcast cancel rollout: resend/resend-monorepo#8126, resend/resend-openapi#85, resend/resend-node#1059, resend/resend-docs#1722, resend/resend-go#144, resend/resend-java#122, resend/resend-php#135, resend/resend-python#250.

Test plan

• ruby -c on touched files — syntax OK
• Could not run the RSpec suite locally (gemspec requires Ruby >= 3.2; this machine only has system Ruby 2.6.10, no version manager available) — relying on CI to verify

🤖 Generated with Claude Code

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Summary by cubic

Adds Resend::Broadcasts.cancel(broadcast_id) to cancel a queued or scheduled broadcast via POST /broadcasts/:id/cancel. Mirrors Emails.cancel (id-only, no params).