[Variant] ObjectBuilder does not reliably check for duplicates

[scovich]

Describe the bug

When duplicate detection is enabled, ObjectBuilder::try_insert or ObjectBuilder::try_with_field detect duplicate field names, but ObjectBuilder::new_list and ObjectBuilder::new_object do not.

To Reproduce

Just invoke either of those methods with a duplicate key -- no error.

Expected behavior

An error should be flagged.

Additional context

Found as part of

• [Variant] [Pathfinding] Variant builder rollback, variant array builder, and read-only metadata builder #8167

[scovich]

Note: The changes inspired by the pathfinding PR referenced above will naturally fix this issue.

[alamb]

label_issue.py automatically added labels {'parquet-variant'} from #8172