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: Use stack buffer with digit-pair conversion, allocate only 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 perfomance 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