[ryanvickr]

Rationale for this change

Fixes #55487.

DeltaLengthByteArrayEncoder currently:

1. Overwrites caller-provided MemoryPool* arguments in its constructor (pool = ::arrow::default_memory_pool() instead of a default parameter), causing both DELTA_LENGTH_BYTE_ARRAY and DELTA_BYTE_ARRAY's suffix_encoder_ to allocate on default_memory_pool() instead of the caller's pool.
2. Encodes Arrow binary/string lengths one element at a time in PutBinaryArray without pre-reserving sink_ capacity, adding unnecessary per-value overhead and reallocations compared to Put(const ByteArray*, int) and other Parquet writers (such as parquet-rs and Polars).
3. Calls BufferBuilder::UnsafeAppend(nullptr, 0) on empty ByteArray(0, nullptr) values, triggering undefined behavior (memcpy with null source pointer) under UBSan.

What changes are included in this PR?

• Fix DeltaLengthByteArrayEncoder's constructor signature to use MemoryPool* pool = ::arrow::default_memory_pool() as a default argument rather than reassigning pool in the member initializer list.
• Batch length encoding in 256-value chunks in PutBinaryArray, pre-reserve sink_ capacity, bulk-copy contiguous non-null offset-based BinaryArray/LargeBinaryArray payloads in a single UnsafeAppend, and skip zero-byte appends.
• Guard Put(const ByteArray*, int) against calling UnsafeAppend on zero-length items.

Are these changes tested?

Yes, added unit tests in cpp/src/parquet/encoding_test.cc:

• DeltaLengthByteArrayEncodingAdHoc.UsesProvidedMemoryPool verifying allocations across StringArray, nullable StringArray, sliced LargeStringArray, StringViewArray, raw ByteArray, and DELTA_BYTE_ARRAY are tracked on a caller-supplied ProxyMemoryPool.
• DeltaLengthByteArrayEncodingAdHoc.BatchedLengthsSlicesAndZeroLengthSafety covering 256-element batch boundaries (0, 1, 255, 256, 257, 512, 600), non-zero slice offsets on LargeStringArray, StringViewArray, empty/all-null arrays, and ByteArray(0, nullptr) inputs.

Are there any user-facing changes?

Callers passing a custom MemoryPool* when writing DELTA_LENGTH_BYTE_ARRAY or DELTA_BYTE_ARRAY columns will now have encoder buffer allocations tracked on that pool.

Was AI used for this PR?

In accordance to the AI generation guidelines, please disclose below whether and how AI was used in this PR.

AI coding assistance was used to help draft the PutBinaryArray batching implementation, unit tests, and PR description; all changes were reviewed and verified by the author.

PR code and description written by:

• Human
• AI

Reviewed before submission by:

• Human
• AI
• Not reviewed

• GitHub Issue: [C++][Parquet] Fix DeltaLengthByteArrayEncoder memory pool tracking, Arrow array batching, and zero-length safety #55487

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⚠️ GitHub issue #55487 has been automatically assigned in GitHub to PR creator.

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