fix(parquet): read null fixed-size-list parents

[fallintoplace]

Rationale for this change

Parquet does not store child values for a null fixed-size-list parent, while Arrow requires list_size child slots for every parent. The existing reader assumes every parent consumes exactly list_size values and does not validate the final span.

What changes are included in this PR?

Reconstruct children from valid and null parent runs:

• validate that each valid parent consumes exactly list_size values
• validate that null parents consume no stored child values
• materialize list_size null children for each null parent
• validate the final parent span

When there are no null parents, validate the spans and reuse the decoded child array directly.

Are these changes tested?

• go test ./parquet/pqarrow -run 'TestParquetArrowIO/TestFixedSizeList' -count=1

Are there any user-facing changes?

No API changes. This corrects the reported behavior while preserving the existing ownership and compatibility contracts.