fix(10.1): CR-01 answer a value that does not fit its column with a 422

lagoon.Fill now returns a *lagoon.FillTypeError naming the key when a
requested value cannot be stored in its column (a fraction, exponent or
overflow for an integer field, or a value of the wrong type). The admin
save path maps it to a validation_failed 422 on that field instead of a
500 CapabilityError; genuine capability failures keep the 500.

modules/lagoon/fill_test.go

@@ -3,6 +3,7 @@ package lagoon
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"log/slog"
 	"strings"
 	"testing"
@@ -193,3 +194,38 @@ func TestFillJSONNumber(t *testing.T) {
 		}
 	}
 }
+
+// TestFillTypeErrorNamesTheKey proves a value that does not fit its column
+// comes back as a *FillTypeError carrying the requested key, so a caller can
+// answer it as a validation failure on that field, while a bad model stays a
+// plain error.
+func TestFillTypeErrorNamesTheKey(t *testing.T) {
+	type numbers struct {
+		Year  int     `gorm:"column:year"`
+		Count uint8   `gorm:"column:count"`
+		Title string  `gorm:"column:title"`
+		Price float64 `gorm:"column:price"`
+	}
+	allowed := []string{"year", "count", "title", "price"}
+	cases := map[string]any{
+		"year":  json.Number("1977.5"),
+		"count": json.Number("1e21"),
+		"title": true,
+		"price": "cheap",
+	}
+	for key, value := range cases {
+		var row numbers
+		err := Fill(&row, allowed, map[string]any{key: value}, true)
+		var typed *FillTypeError
+		if !errors.As(err, &typed) || typed.Key != key || typed.Err == nil {
+			t.Fatalf("%s = %v: err = %#v, want *FillTypeError for %s", key, value, err, key)
+		}
+		if !strings.Contains(err.Error(), "lagoon: fill "+key+": ") {
+			t.Fatalf("%s: message = %q", key, err.Error())
+		}
+	}
+	var typed *FillTypeError
+	if err := Fill(numbers{}, allowed, map[string]any{"year": 1}, true); err == nil || errors.As(err, &typed) {
+		t.Fatalf("non-pointer model: err = %v, want a plain error", err)
+	}
+}