test(05-06): add lagoon.Fill fuzz test

- FuzzFill seeds extra and server-owned keys against a fixture struct
- Non-allow-listed fields stay at their pre-Fill values with zero panics

lagoon/fill_fuzz_test.go

@@ -0,0 +1,68 @@
+package lagoon
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+// fuzzFillFixture is the D-07 framework-level target: only name/notes are
+// allow-listed. collection_id, secret, and id must stay at their pre-Fill values.
+type fuzzFillFixture struct {
+	ID           uint    `gorm:"column:id"`
+	Name         string  `gorm:"column:name"`
+	Notes        *string `gorm:"column:notes"`
+	CollectionID uint    `gorm:"column:collection_id"`
+	Secret       string  `gorm:"column:secret"`
+}
+
+func FuzzFill(f *testing.F) {
+	f.Add(`{"name":"ok","notes":"n"}`)
+	f.Add(`{"name":"ok","collection_id":9,"secret":"leak","id":99}`)
+	f.Add(`{"name":null,"notes":null}`)
+	f.Add(`{"collection_id":1,"unknown":true}`)
+	f.Add(`{"name":{"nested":true},"notes":[1,2]}`)
+	f.Add(`{"name":1,"notes":false}`)
+	f.Add(`[]`)
+	f.Add(``)
+	f.Add(`null`)
+	f.Add(`{"name":"x","extra":{"deep":{"x":1}}}`)
+
+	f.Fuzz(func(t *testing.T, raw string) {
+		var requested map[string]any
+		_ = json.Unmarshal([]byte(raw), &requested)
+		if requested == nil {
+			requested = map[string]any{}
+		}
+		requested["collection_id"] = float64(99)
+		requested["secret"] = "pwned"
+		requested["id"] = float64(7)
+
+		row := fuzzFillFixture{ID: 1, CollectionID: 3, Secret: "keep"}
+		if err := Fill(&row, []string{"name", "notes"}, requested, true); err != nil {
+			// Type mismatches on allow-listed keys are errors, not panics.
+			// Non-allow-listed fields must still be untouched.
+		}
+		if row.CollectionID != 3 {
+			t.Fatalf("collection_id mutated to %d", row.CollectionID)
+		}
+		if row.Secret != "keep" {
+			t.Fatalf("secret mutated to %q", row.Secret)
+		}
+		if row.ID != 1 {
+			t.Fatalf("id mutated to %d", row.ID)
+		}
+	})
+}
+
+func TestFuzzFillNilAndEmptyNeverPanic(t *testing.T) {
+	row := fuzzFillFixture{CollectionID: 3, Secret: "keep"}
+	if err := Fill(&row, []string{"name"}, nil, true); err != nil {
+		t.Fatalf("nil map: %v", err)
+	}
+	if err := Fill(&row, []string{"name"}, map[string]any{}, true); err != nil {
+		t.Fatalf("empty map: %v", err)
+	}
+	if row.CollectionID != 3 || row.Secret != "keep" {
+		t.Fatalf("zero-input fill mutated %+v", row)
+	}
+}