Function apply_update 

pub fn apply_update(
    conn: &Connection,
    candidate: &UpdateCandidate,
    action: UpdateAction,
    new_content: &str,
) -> Result<UpdateResult>

Apply action to an existing memory and return the result.

The caller is responsible for passing the new_content that triggered the update; it is used for Replace and Merge actions.

Note: this function does NOT write to update_log itself. Call create_update_log separately so the caller controls reason text.