Function parse_with_substitutions 

pub fn parse_with_substitutions(
    command: &str,
) -> Result<ParsedPipeline, ParseError>

Parse a shell command into a recursive pipeline tree.

Substitutions are recursively parsed: echo $(cmd1 && cmd2) produces a segment whose substitution contains a two-segment pipeline. The tree can be evaluated bottom-up — inner substitutions execute first.

Recursion depth is capped at 32 levels. Deeper nesting produces an empty pipeline with has_parse_errors: true.