Three runtime errors surface when a parallel-workload run drives an
Iceberg sink with a wider mix of column types:

1. "Datum Int16 does not match builder Int32Builder" — Iceberg has no
   smallint, so the writer context arrow schema (derived from the iceberg
   schema) uses Int32 while Materialize rows still carry Datum::Int16.
   Add a lossless Int16 -> Int32 promotion in ArrowColumn::append_datum,
   mirroring the existing UInt16 -> Int32 case.

2. "Field 'value' missing extension metadata" — Materialize names map
   fields entries/keys/values, but iceberg-rust's arrow conversion names
   them key_value/key/value. merge_field_metadata_recursive matched by
   name and silently dropped the value field's extension metadata, so
   ArrowBuilder later failed when constructing the inner builder. Match
   the map entries struct positionally instead.

3. "Failed to create EqualityDeleteWriterConfig: field_id N not found" —
   the planner accepted Range types as Iceberg equality delete keys, but
   ranges lower into Iceberg structs and iceberg-rust's
   RecordBatchProjector skips nested fields, so the equality field id is
   unreachable at runtime. Drop Range from the allow-list so the failure
   is caught at sink creation instead.

builder_for_datatype was hard-coding MapFieldNames::default()
(entries/keys/values) when constructing the MapBuilder, regardless of
what the surrounding Schema actually said. For Iceberg the schema's map
fields are key_value/key/value (preserved by merge_map_entries_metadata),
so the resulting MapArray's nested DataType disagreed with the schema and
RecordBatch::try_new rejected every row — the sink stalled silently and
iceberg_scan saw an empty table.

Read entry/key/value names off the schema's entries struct so the
MapArray matches whichever convention the caller chose. COPY TO S3
PARQUET keeps building its schema with the arrow-rs defaults, so its
output is unchanged.

Also restores the DuckDB iceberg_scan assertion on the map_table sink in
catalog.td now that the round-trip actually works.

…field