## Proposed changes

- Preserve the full Paimon `RowType` arity when reading projected struct
fields.
- Add unit coverage for non-leading, non-contiguous, and complete struct
projections backed by a binary row.

## Problem summary

Struct projection entries use original child indexes, but the scanner
initialized the nested binary row with the number of projected fields. A
projection such as the third child therefore created a one-field row and
then accessed index two, producing incorrect reads or an out-of-bounds
failure.

## Regression coverage

- `testUnpackNonLeadingStructField` constructs a three-field binary row
and projects only original field index 2. The previous implementation
fails with `index (2) should < 1`.
- `testUnpackNonContiguousStructFields` projects original field indexes
0 and 2. The previous implementation fails with `index (2) should < 2`.
- `testUnpackCompleteStruct` protects the existing full-struct behavior.
- With this fix restored, all three tests pass with Maven build caching
disabled.

## Validation

- Reverted the row-arity fix locally and confirmed that the two
pruned-field regression tests fail.
- Restored the fix and reran the same test class: 3 tests, 0 failures, 0
errors.
- Ran all Paimon scanner test classes with Maven build caching disabled:
15 tests, 0 failures, 0 errors.
- Checkstyle passed for the affected reactor modules.