Add custom tracking domain support to the Node.js SDK

[quzhi1]

Summary

• Add optional trackingOptions.domainName to the shared send and draft tracking-options request type.
• Verify wire serialization emits tracking_options.domain_name exactly once and preserves existing payloads when the field is omitted.
• Add regular Send, Transactional Send, draft, and scheduled-send examples, including the distinction between a Transactional sender domain and tracking hostname.
• Add an Unreleased changelog entry.

Why

The shared SendMessageRequest and TrackingOptions types cover regular sends, Transactional Send, draft create/update, and scheduled sends. Adding the optional property to that shared type lets developers select a tracking hostname without type escape hatches or operation-specific serializers.

Developer impact

Developers can supply an active organization-owned custom hostname as trackingOptions.domainName. The SDK serializes it as tracking_options.domain_name. Existing requests are unchanged when the optional property is omitted.

For Transactional Send, the top-level domainName remains the verified sender domain in /v3/domains/{domainName}/messages/send; requestBody.trackingOptions.domainName is the separate tracking hostname used in recipient-visible links and open pixels.

Validation

• npm test -- --runInBand — 30 suites, 484 tests passed
• npm run lint
• npm run lint:prettier:check
• npm run build
• Strict TypeScript check of examples/messages/messages.ts against the built package
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.75%. Comparing base (cd785ad) to head (52b30cd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #758   +/-   ##
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  Coverage   98.75%   98.75%           
=======================================
  Files          43       43           
  Lines        1041     1041           
  Branches       90       90           
=======================================
  Hits         1028     1028           
  Misses         12       12           
  Partials        1        1           

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