chore: remove Array -> Seq round trips flagged by the 2.13 copy deprecation

[andygrove]

Which issue does this PR close?

No issue filed — mechanical compiler-warning cleanup, split out of #5141.

Rationale for this change

Scala 2.13 deprecates the implicit Array → immutable.IndexedSeq conversion because it silently copies. Comet triggers it at 8 sites, plus one related "passing an explicit array value to a Scala varargs method" deprecation, for 9 of the 127 warnings under the 2.13 profiles.

The compiler suggests .toIndexedSeq. That is a no-op change: the deprecated implicit is defined as

implicit def copyArrayToImmutableIndexedSeq[T](xs: Array[T]): IndexedSeq[T] =
  if (xs eq null) null
  else new ArrayOps(xs).toIndexedSeq

so .toIndexedSeq performs exactly the copy that is already happening — it just makes it visible. (The only behavioral difference is null: the implicit yields a null Seq, the explicit call NPEs. None of these nine arrays can be null.) So the copies are all correct today; the question is whether they are needed.

At 7 of the 9 sites they are not — an Array is materialized and then immediately has to become a Seq again, so this PR removes the round trip rather than annotating it.

What changes are included in this PR?

Round trips removed (7 sites):

• CometLocalTableScanExec.unsafeRows is built as an IndexedSeq instead of an Array, since its only consumers are .length and SparkContext.parallelize, which needs a Seq. Drops one copy of the projected rows.
• CometScanExec.setFilesNumAndSizeMetric is private to Comet and only sums file counts/sizes, so it now takes an Array[PartitionDirectory] and selectedPartitions converts the listFiles result once (the .toArray moves from the end of the block onto the listFiles call). Same number of copies as before, two fewer conversions.
• Utils.stringToSeq splits, trims and filters through an iterator, so the two intermediate arrays are gone.
• CometTestBase.internalCheckSparkAnswer holds the collected rows as a Seq[Row], which is what both checkAnswerWithTolerance and checkCometAnswer take.
• Tables.df hands its per-row values array directly to GenericRow — which is what Row.fromSeq wraps it in anyway — removing two array copies per generated row on the TPC-H/TPC-DS data generation path. It also maps over the StructType (itself a Seq[StructField]) rather than over schema.fields, which is what removes the varargs warning.

Copies kept (2 sites): FilePartition.maxSplitBytes and FilePartition.getFilePartitions are Spark APIs that take a Seq and only read from it. createFilePartitionsForNonBucketedScan converts the (small) partition array once with .toSeq and uses that for both — which also means splitFiles, the much larger collection, is built as a Seq and never copied. .toSeq rather than .toIndexedSeq because on 2.12 Array.toSeq wraps instead of copying, and neither form is deprecated.

Alternatives considered

• ArraySeq.unsafeWrapArray, which the deprecation message recommends: scala.collection.immutable.ArraySeq does not exist in 2.12, so this would not cross-build.
• Spark's .toImmutableArraySeq (org.apache.spark.util.ArrayImplicits), which is how Spark itself fixed these: only present in Spark 4.x — I checked the 3.4.3 and 3.5.8 jars and the class is absent — so it would need a shim to save a planning-time copy.
• Retyping CometScanExec.selectedPartitions to Seq[PartitionDirectory], which would eliminate all three conversions in that file: rejected because the bucketed path feeds FilePartition(bucketId, files: Array[PartitionedFile]), so groupBy over a Seq would trade one array copy for one copy per bucket.

How are these changes tested?

No new tests — no intended behavior change.

• Warnings under the default profile (Spark 4.1 / Scala 2.13): 127 → 118, with all 9 of these gone and none added. Verified by diffing the full sorted warning list before and after.
• Scala 2.12 (-Pspark-3.5): 15 → 15 — these warnings do not exist on 2.12, and none were introduced.
• test-compile passes on all five profiles: default, -Pspark-3.4, -Pspark-3.5, -Pspark-3.5 -Pscala-2.13, -Pspark-4.0.
• spotless:check and scalastyle:check pass.
• 207 tests pass across CometExecSuite (covers the bucketed scan, dynamic partition pruning and local table scan paths), ParquetReadV1Suite (V1 scan planning) and CometConfSuite (stringToSeq via the seq-valued configs). Every one of them also exercises the changed CometTestBase helper.
• Tables.scala is only reachable with dbgen/dsdgen present, so it is not covered by CI; it is compile-checked and the GenericRow substitution is what Row.fromSeq expands to.

[mbutrovich]

Thanks @andygrove!