Share Parquet field-ID and name resolution between remap_physical_schema and match_struct_fields #6978

What is the problem the feature request solves?

Parquet field resolution by field ID and by case-insensitive name is implemented twice, once for top-level columns and once for nested struct fields, and the two must be kept in sync by hand:

• remap_physical_schema in schema_adapter.rs (L198-L343): cognitive complexity 67, nesting depth 5, 30 commits in the last year.
• match_struct_fields in parquet_support.rs (L458-L534): the same ID/case-folding resolution for nested fields.

remap_physical_schema mixes four concerns in one function: duplicate-ID validation (L207-228), building the id-to-logical map (L231-241), name folding, and a per-field closure with three precedence stages (ID match, fake-name shadowing, case-insensitive name match). The renamed-field construction (Field::new(logical.name(), ...).with_metadata(..) plus name_map.insert) is copy-pasted between the ID path (L262-272) and the name path (L305-315). The closure uses early returns and mutates name_map, fake_counter and occupied_names, so the precedence rules that mirror Spark's matchIdField are hard to follow or unit test.

Because this file is still patched often for Spark-compatibility fixes, a fix to one path that misses the other could reintroduce a schema-evolution bug for nested columns only.

The surrounding adapter code has the same shape: SparkPhysicalExprAdapterFactory::create (L900-L1088) and SparkPhysicalExprAdapter::rewrite (L1152-L1251) each inline several independent steps (duplicate-column checks, id-duplicate roots, rebase policies, physical-name restore).

Describe the potential solution

1. Extract a field resolver shared by both paths that returns an enum such as {IdMatch, Shadowed, NameMatch, Unchanged}, so each precedence stage can be unit tested against Spark's behaviour.
2. Extract check_duplicate_field_ids, build_id_to_logical, a rename_field helper used by both the ID and name branches, and a small FakeNameGenerator holding occupied_names and fake_counter.
3. Use the shared resolver from match_struct_fields.
4. Split create and rewrite into named steps (e.g. check_duplicate_columns, id_duplicate_roots, resolve_rebase_policies, restore_physical_names), keeping the Spark-reference comments with the extracted helpers.
5. Optionally move the roughly 1,900 lines of tests in schema_adapter.rs into a separate file.

Additional context

Line references are pinned to commit 7d8d9b1.