create_scalar_index: replace=True drops the existing index before some preconditions are validated

[FANNG1]

Problem

create_scalar_index validates preconditions on both sides of the destructive drop_index call. With the default replace=True, any check that runs after the drop destroys the existing index and then fails, leaving the dataset with no index and no rebuild.

lance_ray/index.py:

if replace:
    if _index_exists(dataset, name):
        dataset.drop_index(name)          # ~L618  destructive

fragments = dataset.get_fragments()
if not fragments:
    raise ValueError("Dataset contains no fragments")        # ~L634

if fragment_ids is not None:
    ...
    if invalid_fragments:
        raise ValueError(f"Fragment IDs {invalid_fragments} do not exist in dataset")   # ~L639

Reproduction

lr.create_scalar_index(uri=path, column="id", index_type="BTREE", name="my_idx")
lr.create_scalar_index(
    uri=path, column="id", index_type="BTREE", name="my_idx",
    fragment_ids=[999],       # does not exist
    replace=True,             # default
)
# raises ValueError -- and `my_idx` is now gone

Because name is caller-supplied, reusing an existing index name for a call that fails validation drops an unrelated index.

The column-type check is currently on the correct side of the drop (see #5219), but nothing in the code or tests signals that this ordering is load-bearing, so a future contributor adding a precondition has no reason to put it in the right place.

Suggested fix

Hoist all non-destructive validation above the if replace: block — fragment existence, fragment_ids membership, and the segment-count adjustment — so the drop is the last thing that can happen before work starts.

Add a regression test that locks the invariant down: pre-create an index, call create_scalar_index with replace=True and an input that fails validation, then assert both that it raises and that the original index still appears in describe_indices().

Related

Surfaced while reviewing #5219, which restores large_string support for BTREE. Pre-existing, not introduced there.