Function resolve_prompt 

pub fn resolve_prompt(
    inline: Option<String>,
    file: Option<&Path>,
    stdin: bool,
    stdin_reader: impl FnOnce() -> Result<String>,
) -> Result<Option<String>>

Collapse the three prompt sources into a single optional string.

stdin_reader is injected so tests can drive the stdin path without touching the real stdin. In production main passes a closure that reads from std::io::stdin().

Trailing newline characters (\r, \n, or any mix thereof) are stripped from the end of the resolved string — editors and heredocs routinely append one or more, and the AI tool doesn’t want them. If the resolved string is empty or whitespace-only after stripping, None is returned so downstream code behaves as if no prompt was given (avoids passing an empty argv entry like claude "").