Support nullable Iceberg partition writes

[osipovartem]

Summary

• Keep NULL partition tuples through Arrow batch grouping, Parquet data-file metadata, and partition paths.
• Mark contains_null in manifest summaries and treat missing bounds conservatively during manifest selection and file splitting.
• Expose nullable snapshot partition bounds from one manifest-list read so downstream MERGE pruning can retain NULL target rows. This does not change the existing bounds API.
• Preserve the no-NULL vectorized extraction path.

Verification

• In-memory DataFusion/Iceberg integration: NULL-only, non-NULL, and mixed-batch INSERTs; inspect data-file partition tuples and manifest summaries; reload the table from the catalog and query with IS NULL and equality predicates. Assert nullable snapshot bounds for all-NULL and mixed manifests.
• Focused partition, manifest, and geometry unit tests; existing partitioned and TIMESTAMPTZ INSERT regressions.
• cargo clippy -p iceberg-rust -p datafusion_iceberg --all-targets -- -D warnings, fmt, and diff-check passed.
• Local debug benchmark of no-NULL key extraction (base 988ea41a vs PR, separate build targets): Int64 6.1M vs 6.4M rows/s; TIMESTAMPTZ 6.2M vs 6.4M rows/s. This detects no obvious regression but is not a release benchmark.
• Independent read-only review approved correctness, performance fast path, and tests.
• Downstream Rustice#564 passes NULL-partition INSERT/UPDATE/DELETE/MERGE tests against this pinned commit.

API note

Public partition-value helpers now represent nullable values as Option<Value>; external Rust callers need to migrate. This PR does not claim NULL-partition DELETE coverage in this fork itself.