fix: report native shuffle read metrics

[peterxcli]

Which issue does this PR close?

Part of #3996.

Rationale for this change

The direct native shuffle-read path bypasses Spark's normal decode path, so its records-read metric was not updated and temporary shuffle-read metrics were not merged into task metrics. This left the Spark Stage and SQL UIs with missing or zero native shuffle-read values.

What changes are included in this PR?

• Pass the existing SQL shuffle-read metrics callback through CometShuffleBlockIterator.
• Update Spark's records-read metric after each successfully decoded native shuffle batch.
• Merge temporary shuffle-read metrics when a direct native shuffle task completes.
• Extend task-metrics regressions to require the direct native ShuffleScan path, assert SQL/Stages UI aggregates, and preserve metrics on failed attempts.

How are these changes tested?

• cargo build
• cargo fmt --all -- --check
• ./mvnw test -Dtest=none -Dsuites='org.apache.spark.sql.comet.CometTaskMetricsSuite native shuffle read' -Dscalastyle.skip=true (2 tests)
• verified with a built Comet JAR on a fresh Spark 4.1.2 standalone cluster with two workers:
  • executor logs confirmed ShuffleScanExec
  • the Stage UI showed Shuffle Read Size / Records: 2004.6 KiB / 500000
  • the SQL UI showed 500,000 records, 2004.6 KiB read, and 32 local blocks

[comphead]

I'm on it today, thanks @peterxcli

[comphead]

Thanks @peterxcli would you mind to attach a screenshot on what is changed? SparkUI shows currently records from shuffle read stages, just wondering what part of metrics is addressed