fix(index): keep scalar indexes on a LIKE with a mixed string literal

[wombatu-kun]

Supersedes #7310.

DataFusion's type coercion promotes both operands of a LIKE to a common string type, so a Utf8 column matched against a Utf8View or LargeUtf8 pattern arrives as Cast(column, <wider>) LIKE <wider>. maybe_indexed_column recognises only a bare column reference, so index discovery gives up and the predicate falls back to a full scan with a refine: the results stay correct and the index is silently lost. Comparisons are unaffected because DataFusion's simplifier unwraps the column cast for BinaryExpr and InList, and Lance's own resolve_expr covers the same three shapes, but neither has a rule for Expr::Like, so the coercion handling added in #6985 never reached it. That same cast is also what keeps the Utf8View prefix support merged in #7351 from being reachable.

Index discovery now looks through a cast whose source and target are both plain string types. The three string encodings carry the same values in the same order, so the index still answers the original predicate exactly, which matters because an exact answer skips the recheck; a cast that leaves the string types, or one over a non-string indexed column, is still refused. visit_like_expr now also coerces the pattern to the indexed column's type, which every other visitor already does through maybe_scalar. Without that, looking through the cast would let a LargeUtf8 pattern reach a Utf8 index and emit a LikePrefix(LargeUtf8) bound, which the zone map cannot compare against its Utf8 statistics (pruning every zone) and which trips the BTree's physical LIKE assertion that both sides share a type.

apply_regex_flags gains the Utf8View variant: regexp_like has a per-argument coercible signature that never unifies its arguments, so its flags literal reaches the parser in whatever variant the caller built. The tests drive Planner::create_filter_plan rather than the parsers directly, and the LIKE-prefix case is parameterised over the column type as well as the literal variant, because the two hazards sit in different cells of that matrix.

[lance-gatekeeper]

✅ Gate recommendation: approve.

The planner now preserves scalar-index discovery only across the value- and ordering-preserving Utf8, LargeUtf8, and Utf8View casts, then normalizes LIKE bounds to the indexed type before parser dispatch. Non-string casts still fall back to refinement, and exact BTree paths cannot receive a mismatched bound.

[wombatu-kun]

The windows failure here is unrelated to this change: pytest-xdist workers race on the shared torchinductor compile cache, and the same PermissionError reproduces on main at the identical cache path. Filed as #9055.

The rest of the matrix is green. I re-ran it by reopening the PR, which is why the timeline shows a close/reopen pair.