Refactor run_ninja into focused helper functions to reduce complexity

[sourcery-ai]

The run_ninja function has grown complex and would benefit from refactoring to improve readability and maintainability. Currently, it handles command construction, logging, and output streaming all within a single function, which increases cognitive load and makes testing more difficult.

Suggested Refactor:
Split run_ninja into three focused helper functions:

1. Command-builder: Responsible for assembling the command arguments, working directory, job count, and build file.
2. Command-logger: Handles redacting and logging the final command invocation.
3. Output-streamer: Spawns threads to pump stdout and stderr, and returns when both finish.

This would allow run_ninja to act as a simple orchestrator, improving clarity and testability. Example structure:

pub fn run_ninja(
    program: &Path,
    cli: &Cli,
    build_file: &Path,
    targets: &BuildTargets<'_>,
) -> io::Result<()> {
    let mut cmd = build_ninja_command(program, cli, build_file, targets)?;
    log_command(&cmd);

    let mut child = cmd.spawn()?;
    let (out_handle, err_handle) = stream_output(child.stdout.take().unwrap(), child.stderr.take().unwrap());

    let status = child.wait()?;
    out_handle.join().ok();
    err_handle.join().ok();

    if status.success() {
        Ok(())
    } else {
        Err(io::Error::new(
            io::ErrorKind::Other,
            format!("ninja exited with {status}"),
        ))
    }
}

Each helper function would be self-contained and easily testable. See the original suggestion for example implementations.

Action Items:

• Refactor run_ninja into three helper functions as described above.
• Ensure each helper is unit tested.
• Confirm that all existing functionality is preserved after refactoring.

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I created this issue for @leynos from #152 (comment).

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