Function generate_fast_commit 

pub async fn generate_fast_commit(
    stat: &str,
    diff: &str,
    model_name: &str,
    scope_candidates_str: &str,
    user_context: Option<&str>,
    config: &CommitConfig,
    debug_dir: Option<&Path>,
) -> Result<ConventionalCommit>

Generate a complete commit in a single API call (fast mode).

Returns a ConventionalCommit directly — no separate summary phase.