GH-49420: [C++][Gandiva] Fix castVARCHAR memory allocation and len<=0 handling

[dmitry-chirkov-dremio]

Rationale for this change

The castVARCHAR functions in Gandiva have memory allocation inefficiencies and missing edge case handling. See GH-49420 for details.

What changes are included in this PR?

Functional fixes:

• bool: Remove unused 5-byte arena allocation; return string literal directly
• int32/int64: Add handling for len=0 (return empty string) and len<0 (set error)

Memory allocation optimizations:

• int32/int64: Allocate fixed small buffer (11/20 bytes) directly in arena, use optimized digit-pair conversion writing right-to-left, then memmove to align. Returns min(len, actual_size) bytes.
• date64: Allocate only min(len, 10) bytes upfront (output is always "YYYY-MM-DD")
• float32/float64: Allocate only min(len, 24) bytes upfront (max output length)

Code cleanup:

• Extract common code into helper macros to reduce duplication

Are these changes tested?

Yes. Added tests for len=0 and len<0 edge cases for int64, date64, float32, float64, and bool types. All existing Gandiva tests pass. Adhoc performance benchmarking was performed both via direct expression evaluation as well as via query execution via Dremio.

Are there any user-facing changes?

No. Users will see reduced memory usage and proper error messages for invalid len parameter values.
Note: Error messages for negative len remain different between precompiled ("Output buffer length can't be negative") and interpreted ("Buffer length cannot be negative") code paths, preserving existing behavior.

• GitHub Issue: [C++][Gandiva] Fix castVARCHAR memory inefficiencies, unused bool allocation, and missing len<=0 handling #49420

[dmitry-chirkov-dremio]

Let me go through first-timer's hurdles (like pre-commit clang format failures).
GH-49347 for 'aws/core/utils/pagination/Paginator.h' file not found

[dmitry-chirkov-dremio]

Pushed clang-format fixes

[kou]

@lriggs @akravchukdremio @xxlaykxx You may want to review this.

[dmitry-chirkov-dremio]

@lriggs @akravchukdremio @xxlaykxx You may want to review this.

https://github.com/telemenar?target=https://github.com is reviewing offline. Second commit is based on their feedback.

[dmitry-chirkov-dremio]

Performance Benchmark Results

Benchmarks run on Apple M3, 5 repetitions each. Tests exercise castVARCHAR for Int32/Int64 with various len parameters. We see small yet noticeable improvements for int conversion. Date conversion's performance remained as is.

Test data:

• Int32 / Int64: Full-range random integers (1M rows)
• Int32Small / Int64Small: 2-digit integers 10-99 (1M rows)

Int32 (Full-Range Random)

len	Original (μs)	Optimized (μs)	Δ
1	17,200	16,856	-2.0% ✅
11	18,856	18,977	+0.6%
100	19,088	18,819	-1.4% ✅
65536	18,792	18,951	+0.8%

Int32 (Small 2-Digit)

len	Original (μs)	Optimized (μs)	Δ
1	11,791	11,062	-6.2% ✅
11	12,461	11,756	-5.7% ✅
100	12,606	11,878	-5.8% ✅
65536	12,353	11,912	-3.6% ✅

Int64 (Full-Range Random)

len	Original (μs)	Optimized (μs)	Δ
1	17,801	17,246	-3.1% ✅
20	19,785	19,010	-3.9% ✅
100	19,268	19,439	+0.9%
65536	19,688	18,950	-3.7% ✅

Int64 (Small 2-Digit)

len	Original (μs)	Optimized (μs)	Δ
1	11,747	11,255	-4.2% ✅
20	12,581	11,435	-9.1% ✅
100	12,357	11,728	-5.1% ✅
65536	12,586	11,796	-6.3% ✅

Memory savings: Allocates 11-20 bytes per integer instead of up to 65,536 bytes - significant for engines that tend to use VARCHAR(65536)