Restore Variant object field offsets after failed append (v59)

[osipovartem]

Backport of the standalone upstream fix in apache#11403 to the Arrow revision used by Rustice. Validated duplicate keys are rejected before mutating the field map, and a failed permitted overwrite restores the previous field offset. The default non-validating path keeps its original fields.insert operation.

Validation: 159 parquet-variant and 333 parquet-variant-compute unit tests pass; format and library Clippy pass (one unrelated Arrow warning). Quick builder benchmarks varied substantially between runs under WSL; the representative default path keeps the original single map insertion. Independent read-only review approved correctness, regressions, performance approach, API compatibility and test coverage.

Follow-up: update Rustice's pinned Arrow revision after merge.

[osipovartem]

CI note: this PR changes only parquet-variant/src/builder/object.rs. The failing jobs are in unchanged files: cargo fmt reports parquet/src/encodings/decoding.rs, rustdoc reports arrow-avro redundant links, and the Linux/macOS/Windows test jobs fail parquet/tests/arrow_reader/bad_data.rs::non_standard_delta_blocks (expected an error, got a batch). Focused local parquet-variant tests and clippy pass. These failures appear unrelated to this one-file change.