feat: add broadcasts.recipients() method - #261

Adds Broadcasts.recipients() / recipients_async() for GET /broadcasts/{id}/recipients.

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

Tests: 584 passed. flake8 + mypy clean.

Summary by cubic

Adds resend.Broadcasts.recipients() and resend.Broadcasts.recipients_async() to list a broadcast’s recipients by event type with filters and cursor pagination. Previously recipient-level delivery and engagement data was not available; now clients can query delivered, opened, clicked, bounced, and more, including counts and clicked links when applicable.

• Public API: adds RecipientsParams and RecipientsResponse, plus BroadcastRecipient, BroadcastRecipientEventType, BroadcastRecipientBounceType, and BroadcastRecipientClickedLink (exported via resend.__init__).
• Filtering and pagination: requires type; supports email, bounce_type (only when type="bounced"), limit (1–100), and cursor params after/before (mutually exclusive). broadcast_id goes in the path; other params go in the query.
• Response shape: returns object="list", data: List[BroadcastRecipient], and has_more. Optional fields: count for opened/clicked, bounce_type for bounced, and clicked_links for clicked.
• Examples and tests: adds sync/async examples; tests cover success cases, pagination, optional fields, 404 errors, and empty content errors.

^{Written for commit 4abdc68. Summary will update on new commits.}