fix(10.1-03): fill numeric model fields from JSON numbers

- lagoon.Fill converts a json.Number (from a UseNumber decoder, as cabana's
  save path uses) into integer, unsigned and float fields; a fraction or an
  overflow is an error
- before this, saving a type: number field into an *int column was a 500
- README documents the conversion

modules/lagoon/fill.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log/slog"
 	"reflect"
+	"strconv"
 	"strings"
 	"sync"
 )
@@ -159,12 +160,49 @@ func convertValue(src reflect.Value, destType reflect.Type) (reflect.Value, erro
 	if src.Type().AssignableTo(destType) {
 		return src, nil
 	}
+	if n, ok := src.Interface().(json.Number); ok {
+		if out, handled, err := convertNumber(n, destType); handled {
+			return out, err
+		}
+	}
 	if src.Type().ConvertibleTo(destType) {
 		return src.Convert(destType), nil
 	}
 	return reflect.Value{}, fmt.Errorf("cannot assign %s to %s", src.Type(), destType)
 }
 
+// convertNumber parses a json.Number, which a request decoder using
+// UseNumber produces, into an integer, unsigned or float field. A value that
+// does not parse as that kind or overflows it is an error. handled is false
+// for any other destination kind, so a string field still takes the number's
+// text through the ordinary conversion.
+func convertNumber(n json.Number, destType reflect.Type) (reflect.Value, bool, error) {
+	out := reflect.New(destType).Elem()
+	switch destType.Kind() {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		i, err := strconv.ParseInt(n.String(), 10, 64)
+		if err != nil || out.OverflowInt(i) {
+			return reflect.Value{}, true, fmt.Errorf("cannot assign number %s to %s", n, destType)
+		}
+		out.SetInt(i)
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
+		u, err := strconv.ParseUint(n.String(), 10, 64)
+		if err != nil || out.OverflowUint(u) {
+			return reflect.Value{}, true, fmt.Errorf("cannot assign number %s to %s", n, destType)
+		}
+		out.SetUint(u)
+	case reflect.Float32, reflect.Float64:
+		f, err := strconv.ParseFloat(n.String(), 64)
+		if err != nil || out.OverflowFloat(f) {
+			return reflect.Value{}, true, fmt.Errorf("cannot assign number %s to %s", n, destType)
+		}
+		out.SetFloat(f)
+	default:
+		return reflect.Value{}, false, nil
+	}
+	return out, true, nil
+}
+
 // encryptedFromRequest treats request input for an Encrypted column as
 // plaintext. It never falls through to Encrypted.Scan: Scan decrypts, so a
 // write path that scanned request input would reject real secrets and accept