Which issue does this PR close?

Closes #4774.

Rationale for this change

Comet silently produces wrong results when the same Iceberg table is scanned more than once with different pushed-down filters (for example a FULL OUTER JOIN or UNION ALL of two differently-filtered reads). Spark's non-AQE ReuseExchangeAndSubquery keys exchange reuse off Exchange.canonicalized, which delegates to the scan's canonical form. CometIcebergNativeScanExec.equals/hashCode/doCanonicalize identified a scan by metadataLocation, output, serializedPlanOpt, and runtimeFilters only. None of these carry the pushed static filters, which live in the @transient originalPlan that canonicalization nulls out. Two scans of the same table+snapshot with different filters therefore canonicalize identically, and reuse collapses them into one, so one branch reads the other branch's data.

Vanilla Spark avoids this because BatchScanExec.equals compares scan.toBatch, and Iceberg's SparkBatch.equals compares table.name() plus SparkScan.hashCode(), which folds in pushed filters, snapshot, branch, and read schema. Spark also keeps the scan reference alive through doCanonicalize, so the fingerprint survives. Comet's V1 Parquet scan (CometNativeScanExec) is not affected because its filters are top-level partitionFilters/dataFilters fields that are already in equality and preserved by canonicalization.

What changes are included in this PR?

• Add a non-transient scanHashCode: Int field to CometIcebergNativeScanExec, captured from scanExec.scan.hashCode() in the apply factory while the transient scan is still available.
• Include scanHashCode in equals and hashCode, and carry it through doCanonicalize (it is the only distinguishing field left once originalPlan is nulled) and convertBlock.

How are these changes tested?

New test in CometIcebergNativeSuite: "exchange reuse must not collapse scans with different pushed filters (#4774)". It runs a UNION ALL of two aggregations over the same partitioned Iceberg table with different partition filters under non-AQE with spark.sql.exchange.reuse=true. It fails on main (branch B reuses branch A's exchange, yielding A twice and dropping B) and passes with this change. Beyond checkSparkAnswerAndOperator, it asserts both CometIcebergNativeScanExec nodes survive and that no ReusedExchangeExec collapsed the branches. The existing AQE DPP broadcast-reuse tests continue to pass, confirming legitimate reuse (identical scans still share an exchange) is preserved.