Add Transactional Send (POST /v3/domains/{domain_name}/messages/send)

[pengfeiye]

License

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Summary

Adds support for Nylas Transactional Send in the Python SDK.

• Client.transactional_send: New resource with send(domain_name, request_body, …) targeting POST /v3/domains/{domain_name}/messages/send and URL-encoded domain_name.
• Models (nylas/models/transactional_send.py): TransactionalSendMessageRequest (required to, single-sender from_ → JSON from) and TransactionalTemplate; reuses existing attachment, tracking, and custom-header types where applicable.
• Behavior: Aligns with grant messages.send for from_ → from, JSON vs multipart over the 3 MB attachment threshold, and base64 encoding of stream attachments for JSON sends.
• Tests: Path/body, domain encoding, attachments, and from / from_ precedence; client property assertion.
• Changelog: CHANGELOG.md Unreleased entry updated.