Allow UISerializer to serialize Polygon locations in addition to Points

[spilth]

Description

This PR updates the UI Serializer's GeometrySchema to handle Polygon features.

It addresses the bug in #1918

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've added relevant test cases.
• I've added relevant documentation.
• I've marked translation strings.
• I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• I've NOT included third-party code (copy/pasted source code or new dependencies).
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Frontend

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[tmorrell]

Thanks for this! I think the tests could be simplified, but otherwise looks like a great fix to an annoying bug.

[tmorrell]

I'm not a fan of having two versions of this, one with points and one with polygons. I know there are some organizational issues around the test data, so others might have better ideas. But I think including both polygon and point in the full_dict_record, and then checking once that they serialize correctly would be simpler but still have the same effect in validating that polygon is working correctly.

[spilth]

I think including both polygon and point in the full_dict_record, and then checking once that they serialize correctly would be simpler but still have the same effect in validating that polygon is working correctly.

Great idea! I will work on that change.

[spilth]

@tmorrell I've consolidated the test cases and fixtures.

[tmorrell]

Looks great! I'll plan on merging in the next few days unless there is feedback from others.