input_file_name() returns empty values above a converted typed Dataset operation #6708

Describe the bug

With spark.comet.convert.typedDataset.enabled=true, input_file_name(), input_file_block_start() and input_file_block_length() return "", -1 and -1 for every row when Spark evaluates them above a Comet operator that reads the converted output of a typed Dataset operation. Spark returns each row's file path and block offsets. The query succeeds, so the wrong values are silent.

This is #6573 for the conversion that #6564 added. CometSparkToColumnarExec fills an Arrow batch from SerializeFromObjectExec, which pulls rows from the scan below the typed operation. By the time the Spark Project evaluates the expressions, the reader has moved on or unset InputFileBlockHolder. The SerializeFromObjectExec case in CometExecRule.transform calls convertTypedDatasetOutput without going through shouldApplySparkToColumnar, so the guard that #6703 adds there does not cover it.

*(2) Project [input_file_name() AS input_file_name()#576, input_file_block_start() AS input_file_block_start()#577L, input_file_block_length() AS input_file_block_length()#578L, value#568L]
+- *(2) CometColumnarToRow
   +- CometFilter [value#568L], (value#568L >= 0)
      +- CometSparkRowToColumnar
         +- *(1) SerializeFromObject [input[0, bigint, false] AS value#568L]
            +- *(1) MapElements ..., obj#566: bigint
               +- *(1) DeserializeToObject assertnotnull(id#558L), obj#564: bigint
                  +- *(1) ColumnarToRow
                     +- FileScan parquet [id#558L] Batched: true, ...

Steps to reproduce

spark.range(9000).repartition(3).write.parquet("/tmp/ifn")

spark.conf.set("spark.comet.convert.typedDataset.enabled", "true")
spark.read
  .parquet("/tmp/ifn")
  .as[Long]
  .map(_ + 1)
  .where("value >= 0")
  .selectExpr("input_file_name()", "input_file_block_start()", "input_file_block_length()", "value")
  .show(3, false)

Comet returns ["", -1, -1, <value>] for all 9000 rows.

Expected behavior

Each row reports the file it was read from, as Spark does. Leaving the typed operation's output unconverted when the plan uses these expressions would also be acceptable.

Additional context

• Reproduced on main at b80bf4e08 with the default Spark 4.1 profile, comparing against the same query with spark.comet.enabled=false. It still reproduces with fix: keep a Spark scan unconverted when the plan reads input_file_name #6703 merged into main.
• The conversion is off by default and is not in a release yet.
• A fix could check CometScanRule.readsInputFileBlock(plan) from fix: keep a Spark scan unconverted when the plan reads input_file_name #6703 in the SerializeFromObjectExec case, next to the partial-reader check, and add the case to the typed Dataset paragraph in docs/source/contributor-guide/adding_a_new_operator.md.