Preserve identity timestamp partition values

[osipovartem]

Summary

• Accept Arrow microsecond timestamp identity partition keys and emit Iceberg Timestamp or TimestampTZ values according to timezone metadata.
• Reject nullable partition batches before splitting so NULL rows are not silently dropped. Full NULL partition writing remains a follow-up.
• Add focused unit tests for both timestamp kinds and nullable Int64/timestamp fail-closed behavior.
• Add an ignored reproducible key-extraction microbenchmark.

Validation

• Focused iceberg-rust partition tests: 2 passed.
• cargo +1.95.0 clippy -p iceberg-rust --lib --tests and fmt passed.
• Rustice end-to-end LTZ YEAR/identity catalog-reload test passed with this fork revision; eight existing PARTITION BY regressions and partition-filter test passed.
• Release key-extraction microbenchmark (100k rows, 1024 keys, 100 iterations): Int64 95.2M rows/s; timestamptz 94.6M rows/s. This is not full write-path or before/after throughput.
• Independent read-only review approved correctness, common-path cost, API compatibility, and tests after resolving its nullable-row finding.