Apply a joi.array.min to non-optional file upload fields

[davidjamesstone]

Proposed change

An issue has been raised where a form was able to be submitted without any files for a required fileuploadfield

This is because an empty array passes the formSchema (joi.array().required()) so the submission looks valid.

This PR adds a default joi.array.min(1) when a min is not explicitly provided through the designer and the field is not optional.

https://defra-digital-team.slack.com/archives/C06E46GB1HA/p1770299635053199

Jira ticket:

https://eaflood.atlassian.net/browse/DF-843

Type of change

• Bug fix
• New feature
• Breaking change
• Misc. (documentation, build updates, etc)

Checklist

• You have executed this code locally and it performs as expected.
• You have added tests to verify your code works.
• You have added code comments and JSDoc, where appropriate.
• There is no commented-out code.
• You have added developer docs in README.md and docs/* (where appropriate, e.g. new features).
• The tests are passing (npm run test).
• The linting checks are passing (npm run lint).
• The code has been formatted (npm run format).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud