perf: avoid redundant string allocation in LIKE prefix pruning (follow-up to #22562)

[alamb]

Which issue does this PR close?

• Follow-up to fix: LIKE 'prefix%' pruning fails on Utf8View and LargeUtf8 columns #22562 (addresses this review comment).

Rationale for this change

#22562 fixes LIKE 'prefix%' pruning on Utf8View/LargeUtf8 columns by casting the synthesized bound literal to the column type via try_cast_literal_to_type. As @alamb noted, that path re-allocates the prefix string: string_literal_as wraps an already-owned String in ScalarValue::Utf8, and the borrowing try_cast_literal_to_type copies it again through to_string().

This PR lets the string be moved into the casted value instead of copied.

What changes are included in this PR?

• try_cast_literal_to_type now accepts either an owned ScalarValue or a &ScalarValue. When an owned value is passed, string-to-string casts move the underlying String rather than re-allocating it.
  • This uses a small IntoScalarCow trait rather than impl Into<Cow<'_, ScalarValue>>, because std has no blanket From<&T> for Cow<'_, T> and ScalarValue lives in another crate (orphan rule), so &ScalarValue can't convert into a Cow directly.
  • All existing &value call sites keep working unchanged.
• string_literal_as (LIKE prefix pruning) passes the owned prefix through and returns None for unsupported target types, so pruning is skipped cleanly instead of emitting a literal whose type does not match the column statistics.

Cast behavior is otherwise identical — only the redundant string copy is removed.

Note

This is stacked on #22562 (its commits are included here). It will need a rebase onto main once #22562 merges, and will conflict in string_literal_as — that's expected and will be resolved then.

Are these changes tested?

Yes — try_cast_literal_to_type gets a new owned-input unit test; existing cast tests and the prune_like_prefix row-group pruning test continue to pass.

Are there any user-facing changes?

try_cast_literal_to_type's signature changes from (&ScalarValue, &DataType) to (impl IntoScalarCow, &DataType), and a new IntoScalarCow trait is added in datafusion-expr-common. Existing callers passing &ScalarValue are source-compatible. No behavior changes.

🤖 Generated with Claude Code

[alamb]

After putzing with this a while. I think the first thing I need to do is to consolidate the cast implemenations. Will do that first and then optii´