feat: add broadcasts.cancel() method

[dielduarte]

Summary

• Cherry-picks broadcasts.cancel() (feat: add broadcasts.cancel() method #1059, 33273b2) from preview-headless-dashboard onto main.
• Cancel is shipping straight to GA (no beta banner, no feature-flag gate — see resend/resend-monorepo#8126 and resend/resend-docs#1722), so it doesn't belong stuck behind the preview-headless-dashboard prerelease line with metrics/recipients. This gets it into the next stable release instead.
• Adds broadcasts.cancel(id), calling POST /broadcasts/:id/cancel, no body. Mirrors emails.cancel().
• Bumps version to 6.19.0 (minor, additive method).

Part of the broadcast cancel rollout: resend/resend-monorepo#8126, resend/resend-openapi#85, resend/resend-docs#1722, resend/resend-go#144, resend/resend-java#122, resend/resend-php#135, resend/resend-python#250, resend/resend-ruby#216, resend/resend-rust#91, resend/resend-dotnet#136.

Test plan

• Clean cherry-pick from preview-headless-dashboard, diff identical to the original 4-file change
• vitest run src/broadcasts/broadcasts.spec.ts — 21/21 passing
• biome check src/broadcasts — clean
• tsc --noEmit — pre-existing node10 moduleResolution deprecation warning on main unrelated to this change (verified via a clean origin/main worktree)

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commit: 909022a

[cubic-dev-ai]

0 issues found across 1 file (changes from recent commits).

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Requires human review: Adds a new public SDK method with a new backend endpoint call and a version bump, expanding the library's API surface in a GA release; the endpoint contract and release decision need human sign-off.

Re-trigger cubic</sub>