Expose structured capacity errors

errors_internal_test.go

@@ -20,3 +20,31 @@ func TestMapPublicErrorPreservesGenerationCancellation(t *testing.T) {
 		t.Fatalf("mapped error=%v, want context.Canceled", err)
 	}
 }
+
+func TestMapPublicErrorTranslatesCapacityError(t *testing.T) {
+	internalErr := &usecase.CapacityError{BackendID: "limited"}
+
+	err := mapPublicError(internalErr)
+	var publicErr *CapacityError
+	if !errors.As(err, &publicErr) || publicErr == nil {
+		t.Fatalf("mapped error=%v, want public CapacityError", err)
+	}
+	if publicErr.BackendID != "limited" {
+		t.Fatalf("mapped backend ID=%q, want limited", publicErr.BackendID)
+	}
+	if !errors.Is(err, ErrCapacityExceeded) {
+		t.Fatalf("mapped error=%v, want ErrCapacityExceeded", err)
+	}
+	if errors.Is(err, ErrInvalidRequest) || errors.Is(err, ErrLLMGenerate) {
+		t.Fatalf("mapped capacity error has an unrelated category: %v", err)
+	}
+	var leakedInternalErr *usecase.CapacityError
+	if errors.As(err, &leakedInternalErr) {
+		t.Fatalf("mapped error exposes internal CapacityError: %v", err)
+	}
+
+	internalErr.BackendID = "changed"
+	if publicErr.BackendID != "limited" {
+		t.Fatalf("mapped backend ID changed with source error: %q", publicErr.BackendID)
+	}
+}