Function deserialize_null_as_default 

pub fn deserialize_null_as_default<'de, D, T>(
    deserializer: D,
) -> Result<T, D::Error>
where
    D: Deserializer<'de>,
    T: Deserialize<'de> + Default,

Serde helper that accepts explicit null as T::default() for backward-compatible checkpoint/diagnostics payloads. Pair with #[serde(default, deserialize_with = "deserialize_null_as_default")] so both missing and null fields degrade to the default instead of failing deserialization. Reused by downstream crates (e.g. vtcode-core snapshots) so the null-tolerance rule cannot drift between copies.