fix: convert Arrow JSON when updating columns via merge/update path

[xloya]

Problem

Updating an Arrow JSON column through fragment.update_columns / the update merge path fails with a type-mismatch error.

Root cause

The HashJoiner right-side stream did not convert Arrow JSON (Utf8) to Lance JSON (LargeBinary/JSONB) before joining.

Fix

Wrap the right-side stream in a JsonConvertingReader that converts Arrow JSON fields to Lance JSON before the join.

Test

Python test_fragment_update_columns_with_json_column plus dataset update tests.

[codecov]

Codecov Report

❌ Patch coverage is 98.50000% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/write/merge_insert.rs	97.47%	2 Missing and 1 partial ⚠️

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