#122 feat: add broadcasts.cancel() method

Summary

• Adds Broadcasts.cancel(id), calling POST /broadcasts/:id/cancel to cancel a queued or scheduled broadcast.
• Mirrors Emails.cancel()'s request shape (empty-string body, same content type).
• Adds CancelBroadcastResponseSuccess model (id + object).

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.

Test plan

• Added testCancelBroadcast_Success following the existing mock-IHttpClient pattern used by testDeleteBroadcast_Success
• Could not run ./gradlew test locally (no JDK available in this environment) — relying on CI to verify compilation and test pass

🤖 Generated with Claude Code

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Summary by cubic

Adds Broadcasts.cancel(id) to cancel queued or scheduled broadcasts via POST /broadcasts/:id/cancel with an empty JSON body, mirroring Emails.cancel().

• New Features
  • Added CancelBroadcastResponseSuccess (id, object) and testCancelBroadcast_Success.
  • Added JavaDoc for cancel() and CancelBroadcastResponseSuccess; removed an unused test helper.