feat(index): record and carry the JSON index target data type

[wjones127]

Populates JsonIndexDetails.target_data_type, added to the format in #9303, so a JSON index's persisted details describe the type its target index was trained on instead of leaving a rebuild to re-infer it.

The recorded type is read back when an index is loaded and copied forward when it is compacted or updated, rather than asked of the target index each time — only BTree targets can answer that, so re-deriving would keep losing the type for bitmap and every other target on every compaction, which is the case the issue is about. It is held as the raw enum value so a variant written by a newer build survives a compaction performed by this one, as the format requires.

Details written before the field existed are upgraded in passing wherever the type can be recovered: always on the update path, which already requires the target to report the type it was trained on, and for BTree targets on the remap path. derive_index_params reports the type so a rebuild reproduces it, and list_indices reports it too.

Not included

JsonIndex::training_data_type is deliberately unchanged, so update on a non-BTree target still fails as before. That guard is load-bearing: BitmapIndex::update takes its value type from the incoming stream and would merge mismatched keys rather than erroring.

uv run make lint was not run: uv sync builds the pylance extension, which needs a local python/Cargo.toml edit in this worktree. ruff and ruff-format were run over the diff via pre-commit instead.

Closes #9256

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[lance-gatekeeper]

✅ Gate recommendation: approve.

The implementation patch is unchanged after the rebase. It records the trained target type, preserves raw values through load, remap, and update—including unknown future variants—and recovers legacy UNSPECIFIED only from the target index, matching the clarified format contract in #9303. Derived rebuild parameters carry the stored type forward.

#9303 remains the prerequisite and should land first.