Preserve string slice function return types

[xudong963]

Which issue does this PR close?

• Closes left, right, and substr always return Utf8View for Utf8 input #23329.

Rationale for this change

left, right, and substr currently always return Utf8View, even when their first argument is Utf8 or LargeUtf8. This makes them inconsistent with nearby string-producing functions such as lower, upper, reverse, translate, and substr_index, which preserve the first argument's string type.

The unconditional Utf8View return type also means callers that intentionally avoid view types cannot keep Utf8 flowing through these functions without explicit casts back to Utf8, even when they explicitly opt out of view types with datafusion.sql_parser.map_string_types_to_utf8view=false and datafusion.execution.parquet.schema_force_view_types=false.

This PR revisits the performance tradeoff from #21441 / #21442 and makes these functions follow the same input-driven return-type contract as the other string functions.

What changes are included in this PR?

• Change left, right, and substr return type inference to use the first argument's string type.
• Return StringArray / LargeStringArray for Utf8 / LargeUtf8 inputs.
• Keep the optimized StringViewArray path for Utf8View inputs.
• Update unit tests and sqllogictests to cover Utf8, LargeUtf8, and Utf8View return types.

Are these changes tested?

Yes. I ran:

cargo fmt --all
cargo test -p datafusion-functions unicode::left::tests::test_functions
cargo test -p datafusion-functions unicode::right::tests::test_functions
cargo test -p datafusion-functions unicode::substr::tests::test_functions
cargo test -p datafusion-functions unicode::substr::tests::test_sliced_string_array_array_args
cargo test -p datafusion-sqllogictest --test sqllogictests -- string/string.slt --test-threads 1
cargo test -p datafusion-sqllogictest --test sqllogictests -- string/string_view.slt string/large_string.slt string/dictionary_utf8.slt --test-threads 1
cargo clippy -p datafusion-functions --lib --tests -- -D warnings

Are there any user-facing changes?

Yes. left, right, and substr now preserve the first argument's string type:

• Utf8 -> Utf8
• LargeUtf8 -> LargeUtf8
• Utf8View -> Utf8View

This may remove implicit downstream casts for users that expect Utf8, but it also means Utf8 / LargeUtf8 inputs no longer use the previous zero-copy Utf8View result representation.