Add Java Variant logical type inspection support

[nartal1]

Description

Closes #23184.

Adds cuDF Java and JNI bindings for the experimental Variant logical type inspection API introduced by #23491.

This PR adds:

• VariantUtils.getVariantTypeId(ColumnView), returning an owning UINT8 column of logical type IDs
• the experimental VariantLogicalType Java enum with explicit values matching libcudf
• a JNI wrapper for cudf::io::parquet::experimental::get_variant_type_id

The Java API follows the libcudf behavior for input nulls, encoded Variant nulls, empty values, unrecognized headers, and header-only classification.

This also adds Java regression coverage for #23361, whose libcudf fix was implemented in #23462, verifying that unsupported direct Variant casts are rejected for empty inputs.

Testing

Added tests in VariantUtilsTest for logical type mappings, null and malformed inputs, empty inputs and API validation.

• VariantUtilsTest: 25/25 passed

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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📥 Commits

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• java/src/main/native/src/VariantUtilsJni.cpp

🚧 Files skipped from review as they are similar to previous changes (1)

• java/src/main/native/src/VariantUtilsJni.cpp

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📝 Walkthrough

Summary by CodeRabbit

• New Features
  • Added experimental support for identifying logical types in Variant values.
  • Added APIs to convert between Variant logical types and native identifiers.
  • Added functionality to retrieve Variant type IDs from byte-list columns.
• Bug Fixes
  • Improved handling of null, empty, sliced, malformed, and all-null Variant inputs.
  • Invalid input types and unsupported conversions now receive appropriate handling.
• Tests
  • Added comprehensive coverage for type mappings, aliases, edge cases, and native ID conversions.

Walkthrough

The change adds Java bindings for Variant logical type IDs. It defines native ID mappings, exposes VariantUtils.getVariantTypeId, updates the JNI bridge, and adds tests for valid, malformed, null, sliced, empty, and invalid inputs.

Changes

Variant type inspection

Layer / File(s)	Summary
Java Variant type contract java/src/main/java/ai/rapids/cudf/VariantLogicalType.java, java/src/main/java/ai/rapids/cudf/VariantUtils.java	Adds the experimental VariantLogicalType enum, native-ID lookup, and the getVariantTypeId API with its native declaration.
JNI Variant type extraction java/src/main/native/src/VariantUtilsJni.cpp	Formats the JNI declaration and native call for Variant type-ID extraction.
Variant type-ID validation java/src/test/java/ai/rapids/cudf/VariantUtilsTest.java	Adds header helpers and tests for logical mappings, malformed values, nulls, slices, empty inputs, invalid inputs, and unsupported casts.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to a6f3d

This PR adds Java and JNI support for Variant logical type inspection with API validation and regression coverage; no actionable merge-blocking risk remains.

Suggested reviewers: ttnghia

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	⚠️ Warning	The regression test for unsupported direct Variant casts addresses behavior outside the linked issue's logical type inspection scope.	Move the unsupported-cast regression test to the owning issue or link the relevant issue to establish its scope.
Docstring Coverage	⚠️ Warning	Docstring coverage is 16.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The PR adds the requested Java API, matching enum, JNI binding, and test coverage for Variant logical type inspection [#23184].
Title check	✅ Passed	The title clearly summarizes the main change: adding Java support for Variant logical type inspection.
Description check	✅ Passed	The description directly explains the Java and JNI bindings, API behavior, tests, and regression coverage in the changeset.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[nartal1]

/ok to test a6f3d5c

[gerashegalov]

LGTM