Harden configuration validation

internal/adapters/storage/factory_test.go

@@ -54,3 +54,35 @@ func TestNewObjectStoreFromConfigNoRemoteBackendConfigured(t *testing.T) {
 		t.Fatalf("NewObjectStoreFromConfig() error = %v, want no-backend error", err)
 	}
 }
+
+func TestNewObjectStoreFromConfigDoesNotInferS3FromProviderFields(t *testing.T) {
+	called := false
+	original := newS3Client
+	t.Cleanup(func() { newS3Client = original })
+	newS3Client = func(_ context.Context, _ s3ClientOptions) (s3API, error) {
+		called = true
+		return &fakeS3API{}, nil
+	}
+
+	_, err := NewObjectStoreFromConfig(context.Background(), &config.Config{
+		Pipeline: &config.PipelineConfig{Storage: config.StorageConfig{
+			Backend: config.StorageBackendLocal,
+			S3:      &config.StorageS3Config{Bucket: "my-archive"},
+		}},
+	})
+	if err == nil || !strings.Contains(err.Error(), "no remote object store backend is configured") {
+		t.Fatalf("NewObjectStoreFromConfig() error = %v, want no-backend error", err)
+	}
+	if called {
+		t.Fatal("NewObjectStoreFromConfig() constructed S3 from incidental provider fields")
+	}
+}
+
+func TestNewObjectStoreFromConfigRejectsUnknownBackend(t *testing.T) {
+	_, err := NewObjectStoreFromConfig(context.Background(), &config.Config{
+		Pipeline: &config.PipelineConfig{Storage: config.StorageConfig{Backend: "s33"}},
+	})
+	if err == nil || !strings.Contains(err.Error(), "unsupported pipeline.storage.backend") {
+		t.Fatalf("NewObjectStoreFromConfig() error = %v, want unsupported-backend error", err)
+	}
+}