[system] parse user_changed_user_id updates without from

[jamalkamaladdin]

Summary

Fixes #235.

PhoneNumberChange.from_update reads from with .get().
old_wa_id and from_user.wa_id are None for a user_changed_user_id update without from.
New fields old_user_id and old_parent_id hold previous_user_id and previous_parent_user_id.
Docstrings in pywa/types/system.py and pywa_async/types/system.py list both new fields.

Type of change

• Bug fix
• New feature
• Documentation update
• Refactor
• Chore / CI

Checklist

• I have read the contribution guidelines in CONTRIBUTING.md.
• The PR has a descriptive title and summary.
• I have added/updated tests where appropriate.
• I have run the test suite locally and all tests pass.
• I have updated documentation where required.
• I have added an entry to CHANGELOG.md if this change should be recorded.

How has this been tested?

tests/data/updates/system.json: new user_id_change payload without from, shaped after Meta's user_changed_user_id example.
tests/test_updates.py: 6 checks for user_id_change on sync and async clients.

Breaking changes

old_wa_id accepts None.

Related issues / PRs

#235

Reviewer notes

None.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.96%. Comparing base (fb513db) to head (35a04dc).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #236   +/-   ##
=======================================
  Coverage   79.96%   79.96%           
=======================================
  Files          55       55           
  Lines        9487     9489    +2     
=======================================
+ Hits         7586     7588    +2     
  Misses       1901     1901           

☔ View full report in Codecov by Harness.
📢 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.