Backport named_struct dialect-aware unparsing

[friendlymatthew]

Backports apache#25661 onto the DataFusion commit currently used by Platform.

DataFusion emits named_struct as dictionary syntax for dialects that cannot parse it, breaking SQL round trips. This emits named_struct(...) for those dialects while preserving dictionary syntax where supported.

Supersedes #78 with a single upstream-linked backport commit. The only compatibility adaptation keeps the DF55 branch's chunks_exact(2) iteration in place of current Apache main's as_chunks::<2>().

Validation:

• cargo test -p datafusion-sql
• cargo clippy --all-targets --all-features -- -D warnings

[adriangb]

Thanks. Let's first fix / get apache#25661 merged.