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.go

@@ -25,9 +25,25 @@ type HasHidden interface {
 
 var droppedKeys sync.Map
 
+// FillTypeError reports a requested value that cannot be stored in its
+// column: a fraction, an exponent or an overflow for an integer field, or a
+// value of the wrong type. Key is the column the value was requested for, so
+// a caller can answer it as a validation failure on that field. Fill returns
+// any other failure, such as a model that is not a struct pointer, as a
+// plain error.
+type FillTypeError struct {
+	Key string
+	Err error
+}
+
+func (e *FillTypeError) Error() string { return "lagoon: fill " + e.Key + ": " + e.Err.Error() }
+
+func (e *FillTypeError) Unwrap() error { return e.Err }
+
 // Fill copies requested keys onto model only when they are also in allowed.
 // Unknown and non-fillable keys are dropped with no error (D-06). In
-// non-production, each type+key pair is logged once.
+// non-production, each type+key pair is logged once. A value that does not
+// fit its column is a *FillTypeError.
 func Fill(model any, allowed []string, requested map[string]any, production bool) error {
 	if model == nil {
 		return fmt.Errorf("lagoon: fill model is nil")
@@ -53,7 +69,7 @@ func Fill(model any, allowed []string, requested map[string]any, production bool
 			continue
 		}
 		if err := setField(field, val); err != nil {
-			return fmt.Errorf("lagoon: fill %s: %w", key, err)
+			return &FillTypeError{Key: key, Err: err}
 		}
 	}
 	return nil