[PR] fix: bound Parquet Variant nesting depth [datafusion-comet]

["manuzhang (via GitHub)" <gi...@apache.org>]

manuzhang opened a new pull request, #7015:
URL: https://github.com/apache/datafusion-comet/pull/7015

   ## Which issue does this PR close?
   
   No existing GitHub issue. This fixes unbounded recursion in native Parquet Variant normalization.
   
   ## Rationale for this change
   
   A deeply nested Parquet Variant value can drive recursive native validation and reconstruction until the executor thread exhausts its stack. Native scans now support at most 64 object/list containers along a value path and return `MALFORMED_VARIANT` for deeper input. Typed containers and residual values share the same budget.
   
   ## What changes are included in this PR?
   
   - Check residual nesting iteratively with shallow Variant iteration before recursive validation and unshredding.
   - Bound storage-schema traversal before recursive casts and VariantArray construction.
   - Make object-key compatibility traversal iterative and validate the depth before recursive re-encoding.
   - Document the native nesting limit.
   
   ## How are these changes tested?
   
   Focused native Variant tests on macOS ARM64 with Rust 1.95.0:
   
   ```sh
   CARGO_PROFILE_DEV_DEBUG=0 CARGO_PROFILE_TEST_DEBUG=0 CARGO_BUILD_JOBS=4 \
   cargo +1.95.0 test --locked --offline -p datafusion-comet --lib \
     parquet::cast_column::variant::tests --no-default-features
   ```
   
   Before rebasing onto current main: 21 passed; 0 failed; 1 existing benchmark ignored. The rebase preserves the patch exactly; the three affected files and dependency manifests were unchanged upstream. New regressions cover objects and lists at the limit and one beyond, residual/typed/mixed paths, and 4,096-level residuals rejected on a 2 MiB stack. Existing compatibility and buffer-reuse tests also pass.
   
   `cargo fmt --all -- --check` and `git diff --check` pass. Spark SQL coverage is requested through the `run-spark-4.1-tests` CI label.
   
   


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