feat(broadcasts): add recipients endpoint

[dielduarte]

Adds BroadcastsSvc::recipients() for GET /broadcasts/{id}/recipients.

Spec: resend/resend-openapi#97
Node reference: resend/resend-node#1078

Tests pass. clippy + fmt clean.

---

Summary by cubic

Adds recipients listing for broadcasts so clients can inspect sent/delivered/opened/clicked/bounced/complained/unsubscribed/suppressed recipients. Previously unavailable; now exposed via a typed, cursor‑paginated endpoint.

• Endpoint: GET /broadcasts/{id}/recipients via BroadcastsSvc::recipients(broadcast_id, ListRecipientsOptions); requires an event type; supports optional email and bounce_type filters; before/after cursors with limit default 20 (max 100).
• Types: BroadcastRecipient includes id (opaque cursor), optional contact_id, email; count for opened/clicked; bounce_type for bounced; clicked_links for clicked. Adds BroadcastRecipientEventType and BroadcastRecipientBounceType; re‑exported in src/lib.rs.
• Testing: adds JSON parsing coverage; ignores recipients_not_found because it requires RESEND_API_KEY and network access.

^{Written for commit 21bdbc9. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 2 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[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 the recipients endpoint and supporting data structures to BroadcastsSvc. This expands the public API surface with new types for events, bounces, and pagination options.

Re-trigger cubic</sub>

[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: This PR adds a new public endpoint and several data types for listing broadcast recipients. Human review is required to verify the naming, ergonomics, and correctness of the new API surface against the Resend specification.

Re-trigger cubic</sub>