Refactor splitPart function to reduce memory allocation

[rimijoker]

Summary

Refactor splitPart to reduce memory allocations during string parsing/splitting operations.

Motivation

The previous implementation of splitPart created many short-lived intermediate objects (substrings / temporary buffers), which increased GC pressure and decreased throughput under high load. This refactor reduces those allocations to improve performance and memory efficiency in hot code paths.

What changed

• Rewrote splitPart to avoid creating intermediate String objects and temporary buffers where possible.
• Uses index-based parsing / reuses buffers (or an existing reusable structure) to produce the split parts without unnecessary allocations.
• Preserves the original behavior and public contract of splitPart.