Fix(additional-entity) allow custom properties to create entities list

[Sujanadh]

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation
• 🧑‍💻 Refactor
• ✅ Test
• 🤖 Build or CI
• ❓ Other (please specify)

Related Issue

• Issue Frontend implementation for multiple feature selection within single form submission #1800

There were issues with custom additional entities geojson properties which weren't supported by odk-central failing the creation of entities and dataset properties. Some keywords of odk-central such as uuid, name, label are not supported by odk-central to avoid duplicate entries.

Describe this PR

This PR fixes the mentioned issue where valid properties are prepared from the uploaded additional entity geojson.

changes

• Custom properties are filtered in one step to include only valid, sanitized properties free of reserved keywords. Invalid properties (having invalid characters such as ":") are excluded to prevent API errors.
• Any default property missing from the custom list is appended to ensure dataset requirements for fmtm in ODK Central.

Screenshots

N/A

Alternative Approaches Considered

Did you attempt any other approaches that are not documented in code?

Review Guide

Notes for the reviewer. How to test this change?

Checklist before requesting a review

• 📖 Read the FMTM Contributing Guide: https://github.com/hotosm/fmtm/blob/main/CONTRIBUTING.md
• 📖 Read the HOT Code of Conduct: https://docs.hotosm.org/code-of-conduct
• 👷‍♀️ Create small PRs. In most cases, this will be possible.
• ✅ Provide tests for your changes.
• 📝 Use descriptive commit messages.
• 📗 Update any related documentation and include any relevant screenshots.

[optional] What gif best describes this PR or how it makes you feel?

[spwoodcock]

Excellent stuff 👍