Unparse named_struct as a function call for dialects without dictionary syntax

[MRoci]

The unparser renders named_struct as a DuckDB-style dictionary literal ({"a": 1}) for every dialect (named_struct_to_sql in unparser/expr.rs), but the postgres, MySQL, SQLite and BigQuery parsers reject that syntax, so unparsed SQL containing named_struct fails to re-parse under the same dialect it was unparsed for.

Adds Dialect::supports_dictionary_syntax() (default true, matching the dialects whose parsers accept the literal) and falls back to a plain named_struct(...) function call otherwise, which survives the round trip. The check sits inside named_struct_to_sql so the get_field compound-access path at expr.rs:796, which bypasses dialect overrides, is covered too.

Includes a parse -> plan -> unparse -> re-parse test; the full sql_integration suite (573) and lib unparser tests (37) pass, and default-dialect output is unchanged.

Upstream PR to apache/datafusion to follow; this carries the fix until it lands there.