CDD-2983 reject validation with missing is public with override option

[kathryn-dale]

Description

This PR covers step two of the approach documented here: https://ukhsa.atlassian.net/browse/CDD-2983
This PR includes the following:

• Remove default of "True" from is_public field, UNLESS override is switched on
• Default to raise missing field error if is_public field is missed from data
• Update test data to include is_public: true (which updates 1398 files)

Fixes #CDD-2983

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Type of change

Please select the options that are relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Tech debt item (this is focused solely on addressing any relevant technical debt)

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Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests at the right levels to prove my change is effective
• I have added screenshots or screen grabs where appropriate
• I have added docstrings in the correct style (google)

[sonarqubecloud]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud