fix: don't drop field metadata on merge insert path

[westonpace]

There are various ways to write data and several of them are failing for JSON data at the moment because it requires a conversion from logical to physical representation. I'd like to rework this more generally but I want to get the current implementation working first. This fixes one of the merge_insert paths which is currently failing because the field metadata is lost as part of the operation (and so the data appears to be a normal string column and is not converted)

[github-actions]

Code Review

Summary: This PR fixes a bug where field metadata was lost during merge_insert operations by cloning the entire field instead of reconstructing it without metadata. The fix is minimal and correct.

✅ Review Result: Approved

The fix is straightforward and correct:

// Before: Field metadata (including JSON extension type info) was lost
Arc::new(arrow_schema::Field::new(field.name(), field.data_type().clone(), field.is_nullable()))

// After: Full field clone preserves metadata
Arc::new(field.clone())

Test coverage: Comprehensive tests added for JSON data across multiple operations (append, compaction, merge_insert).

No blocking issues found.

[westonpace]

Possibly related to #4831 but I don't think this closes that as I don't have all merge insert paths tested yet (and I know some of them use the Updater which also does not properly convert schemas yet)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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