fix: Concatenate extension arrays and keep concatenated results valid after inputs are disposed

[CurtHagenlocher]

What's Changed

Fixes the three ArrowArrayConcatenator problems reported in #443, plus a related lifetime issue found while fixing them.

• Extension arrays are concatenated. ExtensionType.Accept passes the visitor through to the storage type, so the storage visitor then rejected the Extension type ID. ArrayDataConcatenator.Concatenate now checks for an extension type first. It requires every input to have the same extension name and metadata, concatenates the storage arrays, and puts the extension type back on the result. This path also applies to child arrays, so e.g. a list of GuidArrays works too.
• View results own their data buffers. ConcatenateBinaryViewArrayData now calls Retain() on each variadic data buffer it takes from an input. The result stays valid after the inputs (including SliceShared inputs) are disposed.
• Empty view inputs with data buffers no longer throw. The loop that copies data buffers now skips zero-length inputs, just as ConcatenateViewBuffer already did when counting them. The buffer count and the view buffer indices now match.
• Other results that aliased an input now retain it. The result used to share an input's memory without holding a reference in three cases: a single input to either Concatenate method, a nested child where only one input contributed values (for example, lists where only one has values), and the zero-length child or values array used when every input is empty. These now use Retain() / SliceShared(0, 0).

Note: a single-input Concatenate call now returns a new array that holds its own references, not the input object itself. The caller should dispose it like any other concatenation result.

Tests

Eight new tests in ArrowArrayConcatenatorTests cover extension arrays (plain, nested in a list, mismatched types), view results that outlive their inputs (plain and SliceShared), empty view inputs with data buffers, and results that outlive a single input, a list with one non-empty input, and a run-end encoded array with one non-empty input. All eight fail without the fix. The full Apache.Arrow.Tests suite passes on net8.0, net462 and net472.

Closes #443.

🤖 Generated with Claude Code

[Copilot]

Copilot review overview

🟢 Approval recommended

The reviewed changes are covered by regression tests and have no unresolved blocking issues.

Review effort: Lite
Findings: None

What changed in this PR

Fixes ArrowArrayConcatenator support for extension arrays and preserves result validity after input disposal.

Changes:

• Adds extension-array concatenation and validation.
• Retains view and aliased buffers safely.
• Adds regression tests for lifetime and empty-input cases.

File	Description
test/​Apache.Arrow.Tests/​ArrowArrayConcatenatorTests.cs	Adds extension, view, and lifetime regression tests.
src/​Apache.Arrow/​Arrays/​ArrowArrayConcatenator.cs	Retains data for single-input results.
src/​Apache.Arrow/​Arrays/​ArrayDataConcatenator.cs	Implements extension concatenation and buffer ownership fixes.

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