fix: preserve shell redirections like 2>&1 when splitting commands

[VincentHardouin]

Summary

• Fix OPERATOR_RE to only match & as a command separator when surrounded by whitespace, preventing false matches in shell redirections like 2>&1, 1>&2
• Before: find / -name "*.log" 2>&1 → snip find / -name "*.log" 2>& snip 1
• After: find / -name "*.log" 2>&1 → snip find / -name "*.log" 2>&1
• Adds 4 new tests covering redirection scenarios

Fixes #3