fix(config): map invalid ladder amendments to ErrLadderAmendInvalid

internal/config/ladder_amend.go

@@ -15,6 +15,14 @@ var (
 	ladderAmountPattern    = regexp.MustCompile(`^(0|[1-9]\d{0,13})(\.\d{1,6})?$`)
 )
 
+// ladderAmendValidationError keeps the Validate message for the caller while
+// matching ErrLadderAmendInvalid, so store callers can map every invalid
+// amendment to a client error with errors.Is.
+type ladderAmendValidationError struct{ err error }
+
+func (e ladderAmendValidationError) Error() string   { return e.err.Error() }
+func (e ladderAmendValidationError) Unwrap() []error { return []error{ErrLadderAmendInvalid, e.err} }
+
 type LadderAmendment struct {
 	ExpectedRevision *int64    `json:"expected_revision"`
 	ScheduledDate    time.Time `json:"scheduled_date"`

internal/config/store_postgres_ladder_amend.go

@@ -24,7 +24,7 @@ func (s *PostgresStore) LadderTrancheScope(ctx context.Context, id string) (acco
 
 func (s *PostgresStore) AmendLadderTranche(ctx context.Context, id, accountID, provider, actor string, amendment LadderAmendment) (*LadderAmendmentResult, error) {
 	if err := amendment.Validate(); err != nil {
-		return nil, err
+		return nil, ladderAmendValidationError{err}
 	}
 	if actor == "" {
 		return nil, errors.New("ladder amendment actor is required")

internal/config/store_postgres_ladder_amend_test.go

@@ -0,0 +1,20 @@
+package config
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAmendLadderTranche_InvalidAmendmentMatchesErrLadderAmendInvalid(t *testing.T) {
+	zero := int64(0)
+	store := &PostgresStore{} // validation fails before the database is used
+	_, err := store.AmendLadderTranche(context.Background(), "id", "account", "aws", "actor",
+		LadderAmendment{ExpectedRevision: &zero, ScheduledDate: time.Now(), AmountUSDHr: "0"})
+	require.ErrorIs(t, err, ErrLadderAmendInvalid)
+	assert.Contains(t, err.Error(), "amount_usd_hr must be a positive decimal")
+	assert.NotContains(t, err.Error(), "per-run cap")
+}