feat: legacy multi-RG input adapter for ColumnPageStream (PR-5)

[g-talbot]

Summary

• New LegacyInputAdapter exposes legacy multi-row-group parquet files through PR-5a's ColumnPageStream so PR-6's merge engine doesn't have to special-case them.
• Pattern: one buffered GET → decode via ParquetRecordBatchReaderBuilder → arrow::compute::concat_batches (preserves order — does NOT re-sort) → re-encode as single-row-group via StreamingParquetWriter (set_max_row_group_row_count(num_rows + 1)) → wrap in private InMemoryByteSource → expose via inner StreamingParquetReader.
• Carries forward key_value_metadata (qh.* keys) and RG0 sorting_columns to the consolidated output.
• 4 GiB defensive cap on input file size (MAX_LEGACY_INPUT_BYTES) — production legacy files are well under 1 GiB.
• Triggered when qh.rg_partition_prefix_len == 0 && num_row_groups > 1. (PR-6 implements the dispatch; this PR provides the adapter as one of the trait impls.)

Stack

main
└── gtt/parquet-streaming-base (= main ∪ PR-2 #6384 ∪ PR-4 #6386)
    └── gtt/column-page-stream-trait (PR-5a #6406)
        ├── gtt/legacy-input-adapter ← PR-5 (this PR)
        └── gtt/parquet-page-decoder (PR-6a #6407)
            └── gtt/streaming-merge-engine-merger (PR-6b — next)
                └── gtt/streaming-merge-engine-multi-rg (PR-6c)

Test plan

• test_empty_multi_rg_input — empty 2-RG fixture consolidates to 0 rows
• test_multi_rg_consolidates_to_single_rg — 3-RG fixture → 1 RG, all pages at rg_idx == 0
• test_data_roundtrip_through_adapter — adapter exposes oracle row count + schema
• test_reencode_preserves_arrays_byte_equal — byte-equal data round trip through reencode_as_single_row_group (added on top of agent's initial commit to close a value-corruption test gap)
• test_kv_metadata_preserved — qh.sort_fields and qh.window_start_secs carried through
• test_sorting_columns_preserved — RG0's sorting_columns preserved on the consolidated RG
• test_dict_and_null_columns_preserved — data-page num_values sums match per column for dict + nullable columns
• test_io_failure_surfaces_as_io_error — buffered GET failure surfaces as LegacyAdapterError::Io carrying the original io::Error
• test_satisfies_column_page_stream_trait — drains via &mut dyn ColumnPageStream and confirms idempotent EOF

CI gates locally green: clippy --workspace --all-features --tests with -Dwarnings, nightly fmt --check, cargo doc --no-deps, cargo machete, license headers, log format, typos. 9/9 adapter tests + 401/401 crate tests pass.

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