Rationale for this change

The lpad_utf8_int32_utf8 and rpad_utf8_int32_utf8 functions have performance inefficiency and a potential memory safety issue:

1. Performance: Single-byte fills iterate character-by-character when memset would suffice. Multi-byte fills use O(n) iterations instead of O(log n) with a doubling strategy.
2. Memory safety: When the fill string is longer than the padding space needed, the code could write more bytes than allocated. Fixed preventatively.

What changes are included in this PR?

1. Memory safety fix: Use std::min(fill_text_len, total_fill_bytes) for the initial copy to prevent overflow
2. Fast path: Add single-byte fill optimization using memset
3. General path: Replace character-by-character loop with doubling strategy for multi-byte fills
4. Tests: Add comprehensive tests for the new code paths

Are these changes tested?

Yes. Added tests covering:

• Large UTF-8 fill characters (4-byte emoji, 3-byte Chinese)
• Single-byte fill boundaries (1 char and 65536 char padding)
• Content verification for fill patterns
• Doubling strategy boundaries
• Partial fill scenarios (fill text longer than padding needed)

Are there any user-facing changes?

No.

• GitHub Issue: [C++][Gandiva] Optimize LPAD/RPAD functions: fix memory safety issue and improve performance #49438