Add custom tracking hostname support to Python SDK

[quzhi1]

Summary

• add optional domain_name support to the shared tracking-options request model used by message sends, scheduled sends, draft create/update, and Transactional Send
• deserialize and serialize the response TrackingOptions.domain_name field while omitting it when unset, preserving existing output
• add supplied and omitted-field wire coverage for regular, scheduled, draft, and Transactional request paths
• add a runnable custom tracking hostname example and an unreleased changelog entry

Why

Implements EN-143 for the public tracking_options.domain_name contract. Python SDK users can now select an active organization-owned tracking hostname without untyped escape hatches.

For Transactional Send, the example and API docstring distinguish the verified sender domain in /v3/domains/{domain_name}/messages/send from the custom tracking hostname in the request body.

Developer impact

Callers can add domain_name alongside links, opens, or both:

"tracking_options": {
    "links": True,
    "opens": True,
    "domain_name": "tracking.example.com",
}

Requests that omit the field are unchanged. Hostname canonicalization, ownership, certificate status, and failure handling remain enforced by the API.

Verification

• .venv/bin/pytest -q — 483 passed, 3 deselected
• focused message, draft, and Transactional Send suite — 74 passed
• compile, dependency, whitespace, and diff checks passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.08%. Comparing base (00e3f81) to head (2f78689).
⚠️ Report is 26 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #484      +/-   ##
==========================================
+ Coverage   98.90%   99.08%   +0.17%     
==========================================
  Files          52       65      +13     
  Lines        2293     3074     +781     
==========================================
+ Hits         2268     3046     +778     
- Misses         25       28       +3     

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