Check every model absorbs a null or missing field, mechanically

Invoice Ninja returns `null` for many nominally-string fields and omits
others, so one model that fails on either takes the whole response down with
it. Rather than trusting each field to carry the right serde attribute, this
serialises `Default`, then nulls and separately removes each field in turn,
and requires the result to still decode.

A wrong *type* is deliberately not covered: that is meant to be a hard
error, so it is not something to absorb.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/decode_tolerance.rs

@@ -0,0 +1,127 @@
+//! Goal 2, checked mechanically: every model must absorb a field that is
+//! `null` or absent.
+//!
+//! Invoice Ninja returns `null` for many nominally-string fields and omits
+//! others, so a model that fails on either takes the whole response down with
+//! it. Rather than trusting each field to carry the right serde attribute,
+//! this serialises `Default` (which emits every non-optional field), then for
+//! each field in turn nulls it and, separately, removes it, and requires the
+//! result to still decode.
+//!
+//! A wrong *type* is deliberately not covered: that is meant to be a hard
+//! error, so it is not something to absorb.
+
+use invoiceninjars::models::*;
+use serde::{de::DeserializeOwned, Serialize};
+use serde_json::Value;
+
+/// Returns one line per mutation that failed to decode.
+fn failures<T: Serialize + DeserializeOwned + Default>(name: &str) -> Vec<String> {
+    let base = serde_json::to_value(T::default()).expect("default serialises");
+    let object = base
+        .as_object()
+        .unwrap_or_else(|| panic!("{name}: default did not serialise to an object"));
+
+    let mut out = Vec::new();
+    for key in object.keys() {
+        let mut nulled = base.clone();
+        nulled[key] = Value::Null;
+        if let Err(e) = serde_json::from_value::<T>(nulled) {
+            out.push(format!("{name}.{key} = null: {e}"));
+        }
+
+        let mut removed = base.clone();
+        removed.as_object_mut().unwrap().remove(key);
+        if let Err(e) = serde_json::from_value::<T>(removed) {
+            out.push(format!("{name}.{key} absent: {e}"));
+        }
+    }
+    out
+}
+
+macro_rules! tolerant {
+    ($($ty:ty),+ $(,)?) => {{
+        let mut all = Vec::new();
+        $( all.extend(failures::<$ty>(stringify!($ty))); )+
+        all
+    }};
+}
+
+#[test]
+fn every_model_absorbs_a_null_or_missing_field() {
+    let all = tolerant![
+        Activity,
+        ActivityFeedEntry,
+        ActivityRef,
+        BankIntegration,
+        BankTransaction,
+        BankTransactionRule,
+        Client,
+        ClientContact,
+        ClientGatewayToken,
+        Company,
+        CompanyGateway,
+        CompanyToken,
+        Design,
+        Document,
+        Expense,
+        ExpenseCategory,
+        GroupSetting,
+        Invoice,
+        InvoiceItem,
+        Location,
+        Payment,
+        PaymentInvoice,
+        PaymentTerm,
+        Product,
+        Project,
+        PurchaseOrder,
+        RecurringExpense,
+        RecurringInvoice,
+        Subscription,
+        SubscriptionStep,
+        SystemLog,
+        Tag,
+        Task,
+        TaskScheduler,
+        TaskStatus,
+        TaxRate,
+        User,
+        Vendor,
+        VendorContact,
+        Webhook,
+    ];
+
+    assert!(
+        all.is_empty(),
+        "{} field(s) break decoding on null/absent:\n{}",
+        all.len(),
+        all.join("\n")
+    );
+}
+
+/// A guard on the test itself: if `Default` stopped emitting fields, the loop
+/// above would pass vacuously.
+#[test]
+fn the_tolerance_test_actually_exercises_fields() {
+    let client = serde_json::to_value(Client::default()).unwrap();
+    assert!(
+        client.as_object().unwrap().len() > 10,
+        "Client::default() should serialise many fields"
+    );
+}
+
+/// The mutation really does bite: a field without `null_to_default` fails.
+#[test]
+fn a_bare_non_option_field_would_be_caught() {
+    #[derive(serde::Serialize, serde::Deserialize, Default)]
+    struct Naive {
+        #[serde(default)]
+        name: String,
+    }
+    assert_eq!(
+        failures::<Naive>("Naive").len(),
+        1,
+        "only the null case fails"
+    );
+}