[EN-145] Document custom tracking domains

[quzhi1]

Description

Summary

• Document the existing tracking_options.domain_name hash contract for regular Send.
• Add Transactional Send, draft create/update, and scheduled-send examples.
• Clarify that a Transactional sender domain is distinct from the custom tracking hostname.
• Add request-forwarding and JSON-shape regression coverage.
• Add an unreleased changelog entry linking to the public message-tracking guide.

Why

Implements EN-145. The Ruby SDK already forwards hash-based request bodies, so custom tracking hostnames need documented request shapes and regression coverage rather than a new typed model.

Developer impact

Ruby callers can pass an active organization-owned hostname as tracking_options.domain_name alongside links and/or opens. Requests that omit the field remain unchanged and continue using the default Nylas tracking hostname.

The examples also document draft inheritance, scheduled-send revalidation, and the distinction between the Transactional Send route's sender domain and the body field's tracking hostname.

Validation

• bundle exec rspec spec - 315 examples, 0 failures
• bundle exec rubocop --config .rubocop.yml - 80 files, no offenses
• bundle exec yard doc --no-output - succeeds
• git diff --check - clean

License

I confirm that this contribution is made under the terms of the MIT license and that I have the authority necessary to make this contribution on behalf of its copyright owner.