👌 Default values of extra fields now checked against schema definitions

[chrisjsewell]

Previously, user set defaults were not being checked against schema, which is not intuitive.
As can be seen in this PR, this does not actually break any tests, principally due to the preceding #1644 and #1645

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.80%. Comparing base (4e10030) to head (23a85dd).
⚠️ Report is 240 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1647      +/-   ##
==========================================
+ Coverage   86.87%   88.80%   +1.92%     
==========================================
  Files          56       70      +14     
  Lines        6532     9989    +3457     
==========================================
+ Hits         5675     8871    +3196     
- Misses        857     1118     +261     

Flag	Coverage Δ
pytests	88.80% <100.00%> (+1.92%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ 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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.