#250 feat: add Broadcasts.cancel() and cancel_async()

Summary

• Adds Broadcasts.cancel(id) / cancel_async(id), calling POST /broadcasts/:id/cancel to cancel a queued or scheduled broadcast.
• Mirrors Emails.cancel()'s shape (empty params, id-only).
• Adds Broadcasts.CancelResponse (object + id).

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.

Test plan

• pytest tests/broadcasts_test.py tests/broadcasts_async_test.py — 24/24 passing
• mypy resend/broadcasts/ — clean

🤖 Generated with Claude Code

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Summary by cubic

Adds cancel support for broadcasts via Broadcasts.cancel(id) and cancel_async(id) to stop queued or scheduled sends with POST /broadcasts/:id/cancel, matching Emails.cancel().

• New Features

  • Broadcasts.cancel(id) and cancel_async(id) to cancel queued/scheduled broadcasts.
  • Broadcasts.CancelResponse returning object and id.

• Bug Fixes

  • Added test to ensure cancel_async raises NoContentError on no-content responses.