TT-4385: add CRUDs for transactional send

[googsvg]

Summary

• New SendTransactionalEmailRequest model — mirrors SendMessageRequest with from as a single required EmailName (domain-verified sender)
• New Domains resource with sendTransactionalEmail() — handles both JSON and multipart (large attachments ≥ 3MB)
• Wired nylas.domains() into NylasClient

Notes

Backward compatible — no existing APIs modified.

Testing

8 new tests (serialization, happy path, overrides, large attachment path). Full suite: 348 tests, 0 failures.

Docs

https://developer.nylas.com/docs/reference/api/transactional-send/send-transactional-email/

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.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.72%. Comparing base (ded101e) to head (78193c4).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #311      +/-   ##
============================================
+ Coverage     82.47%   82.72%   +0.25%     
- Complexity      357      364       +7     
============================================
  Files            37       38       +1     
  Lines          1027     1042      +15     
  Branches         95       97       +2     
============================================
+ Hits            847      862      +15     
  Misses          121      121              
  Partials         59       59              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.