Function is_valid_executable_path 

fn is_valid_executable_path(cmd: &str) -> bool

Validate that a tool command is a safe executable path.

Commands must be:

• A relative path starting with ./tools/
• Free of shell metacharacters that could enable injection
• Free of path traversal sequences (..)

This prevents shell injection attacks like cmd = "rm -rf / && echo hacked" and path traversal attacks like cmd = "./tools/../../../bin/sh".