perf: unwrap identity casts in schema adapter to enable Parquet stats pruning

[mbutrovich]

Which issue does this PR close?

Closes #.

Rationale for this change

DataFusion's default PhysicalExprAdapter inserts a CastExpr around every Column reference whenever the logical and physical Arrow Fields differ in any attribute, including metadata-only or nullability-only mismatches. DataFusion itself absorbs this because its PruningPredicate analyzer recognizes its own CastExpr and peels it to resolve the column against parquet statistics.

Comet's SparkPhysicalExprAdapter::replace_with_spark_cast then swaps that CastExpr for a datafusion_comet_spark_expr::Cast because Spark cast semantics diverge from arrow-cast for overflow, null handling, ANSI mode, etc. The Spark Cast is a different PhysicalExpr type that DataFusion's pruning analyzer does not understand, so build_pruning_predicates returns None at file open time and no row groups are pruned. With Spark range-derived schemas (non-nullable logical) read from Parquet (nullable physical), this fires on every column reference, silently disabling row-group and page-index stats pruning.

For identical source and target data types there is no Spark-specific cast semantics to preserve, so the swap costs us pruning for no benefit.

What changes are included in this PR?

SparkPhysicalExprAdapter::replace_with_spark_cast now skips the Spark Cast wrap when the physical and target data types are equal. Unwrapping is safe because a Cast with equal source and target types is a value-level identity (it does not null-strip or enforce non-null), and Arrow field nullability and metadata are informational, not computational.

How are these changes tested?

CometNativeReaderSuite adds a regression test that writes a 1000-row Parquet file with parquet.block.size=1024, asserts more than one row group via ParquetFileReader, then runs SELECT ... WHERE c1 > 500 and asserts the scan's numOutputRows is strictly less than 1000. Without the fix the scan reads all rows; with the fix row groups whose max is at most 500 are pruned.

[andygrove]

Nice catch . Thanks @mbutrovich

[mbutrovich]

On TPC-DS Q99, for example, we can see the effect of these filters now firing.

On main:

PR #4730: