[HSTACK] Fix MAP schema evolution: allow additive value-struct changes

[cochescu]

Problem

When a Delta table's logical schema has evolved additively — e.g. identityMap: Map<String, List<Struct<id>>> in older Parquet files vs Map<String, List<Struct<id, primary, authenticatedState>>> in the Delta log — DataFusion failed with:

Execution error: Cannot cast column 'identityMap' from
  'Map(key_value: Struct(key: Utf8, value: List(Struct(id: Utf8))))' (physical)
to
  'Map(key_value: Struct(key: Utf8, value: List(Struct(id: Utf8, primary: Boolean, authenticatedState: Utf8))))' (logical)

The schema rewriter rejected the cast and the physical plan builder had no handler for MAP-to-MAP evolution.

Fix

datafusion/common/src/nested_struct.rs

• Adds a DataType::Map arm in cast_column that delegates to a new cast_map_column() function.
• cast_map_column casts the inner key_value StructArray through the existing cast_column logic, so missing nullable fields in the value struct are filled with nulls and extra source fields are ignored — the same additive evolution semantics already supported for plain structs.

datafusion/physical-expr-adapter/src/schema_rewriter.rs

• Adds a (DataType::Map, DataType::Map) arm to the compatibility check in try_adapt_physical_expr that validates the inner key_value struct fields via validate_struct_compatibility, allowing additive changes to pass instead of being rejected by can_cast_types.

Tests

• test_cast_map_with_evolved_value_struct — end-to-end cast of a real-world identityMap: Map<String, List<Struct>> where older files have fewer value-struct fields.
• test_validate_map_value_struct_compatibility — unit test for the schema_rewriter compatibility check on Map types.

Notes

The change is purely additive: the two new match arms intercept MAP types before the existing _ => fallback; all other types are unaffected.