794d87d95c
Several recently-added Config fields (data_retention, cloud_ops, conversational_ai, security_ops) were missing #[serde(default)], causing deserialization failures when those sections are absent from config files. Also fixes security field whose #[serde(default)] was accidentally consumed by the backup doc comment. Fixes test failures: agent_config_deserializes and browser_config_backward_compat_missing_section.