Bash command-substitution parsing hardened against untrusted backtick/$() bytes

The safety parser that analyzes bash commands before running them now specifically detects malformed or unaccounted-for backtick and $() command-substitution syntax, such as an empty or overrun backtick body, or an unsupported escape inside one. When it finds bytes in the command that its parse tree can't account for, it now downgrades the command to "too complex" instead of proceeding on a parse it can't fully trust.