Native Iceberg write panics generating the partition path for a timestamptz partition column #5694

Describe the bug

Writing to a table partitioned on a timestamptz column panics in iceberg-rust while generating the data file's partition path:

org.apache.comet.CometNativeException: called `Option::unwrap()` on a `None` value
    at core::option::unwrap_failed
    at iceberg::spec::values::temporal::timestamptz::microseconds_to_datetimetz
    at <iceberg::spec::values::datum::Datum as core::fmt::Display>::fmt
    at <iceberg::spec::values::datum::Datum>::to_human_string
    at <iceberg::spec::transform::Transform>::to_human_string
    at <iceberg::spec::partition::PartitionSpec>::partition_to_path::{closure#0}
    at <iceberg::spec::partition::PartitionSpec>::partition_to_path
    at <...DefaultLocationGenerator as ...LocationGenerator>::generate_location
    at <...RollingFileWriter<ParquetWriterBuilder, DefaultLocationGenerator, ...>>

microseconds_to_datetimetz unwraps a None when converting the partition value to its human-readable form for the directory name, so the task dies with a Rust panic crossing the JNI boundary rather than an error.

Steps to reproduce

Spark 4.1.3, Iceberg 1.11.0, spark.comet.iceberg.write.enabled=true plus the Iceberg Spark SQL test setup from dev/diffs/iceberg/1.11.0.diff:

./gradlew -DsparkVersions=4.1 -DscalaVersion=2.13 -DflinkVersions= -DkafkaVersions= \
  :iceberg-spark:iceberg-spark-4.1_2.13:test --tests '*TestSparkDataFile*' -Pquick=true -x javadoc

Failing cases:

• TestSparkDataFile.testValueConversionPartitionedTable
• TestSparkDataFile.testValueConversionWithEmptyStats

Both write to a table whose partition spec covers a wide set of column types, including timestamptz.

Expected behavior

The partition path is generated for timestamptz partition values the same way iceberg-java generates it, and the write succeeds. Any value the conversion genuinely cannot represent produces an error rather than a panic.

Additional context

Found by turning the two Iceberg write flags on by default in #5677.

The panic is in iceberg-rust rather than in Comet's own code, so the fix may need to go upstream. Worth checking whether the value reaching microseconds_to_datetimetz is itself wrong (a unit or epoch mismatch on the way in) before concluding the conversion is at fault.

Part of #5649.