fix(providers): skip unavailable user providers instead of failing the whole list

A saved user provider whose type is no longer enabled (e.g. its API key was
removed) made GetProviders return an error for the ENTIRE `providers` query —
one stale row blocked all flow creation in the UI ("No available providers").
Skip and log such rows, mirroring how startup already tolerates disabled
default providers. Pre-existing robustness gap, not introduced by this branch.

Verified live on the docker stand: the `providers` query went from a hard
error to returning all 10 enabled providers (the stale minimax row skipped),
and flow creation works again. Adds a side-by-side regression test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/pkg/providers/get_providers_test.go

@@ -0,0 +1,49 @@
+package providers
+
+import (
+	"context"
+	"testing"
+
+	"pentagi/pkg/config"
+	"pentagi/pkg/database"
+	"pentagi/pkg/providers/provider"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// stubTypedProvider reports only its type — enough for Providers.ListTypes(),
+// which is all GetProviders touches before it rejects an unavailable type.
+type stubTypedProvider struct {
+	provider.Provider
+	ptype provider.ProviderType
+}
+
+func (s stubTypedProvider) Type() provider.ProviderType { return s.ptype }
+
+type stubProvidersQuerier struct {
+	database.Querier
+	rows []database.Provider
+}
+
+func (s stubProvidersQuerier) GetUserProviders(context.Context, int64) ([]database.Provider, error) {
+	return s.rows, nil
+}
+
+func TestGetProviders_SkipsUserProviderOfDisabledType(t *testing.T) {
+	pc := &providerController{
+		cfg: &config.Config{},
+		db: stubProvidersQuerier{rows: []database.Provider{
+			{Name: "stale-minimax", Type: "minimax"}, // not in ListTypes() below
+		}},
+		Providers: provider.Providers{
+			"openai-default": stubTypedProvider{ptype: provider.ProviderOpenAI},
+		},
+	}
+
+	got, err := pc.GetProviders(context.Background(), 1)
+
+	require.NoError(t, err, "one user provider of a disabled type must not fail the whole query")
+	assert.Contains(t, got, provider.ProviderName("openai-default"), "enabled providers stay reachable")
+	assert.NotContains(t, got, provider.ProviderName("stale-minimax"), "the unavailable provider is skipped")
+}

backend/pkg/providers/providers.go

@@ -613,7 +613,10 @@ func (pc *providerController) GetProviders(
 	for _, prv := range providers {
 		p, err := pc.NewProvider(prv)
 		if err != nil {
-			return nil, fmt.Errorf("failed to build provider: %w", err)
+			// A saved provider of a now-disabled type must not fail the whole list —
+			// skip it (like startup tolerates disabled defaults) so the rest stay usable.
+			logrus.WithError(err).Warnf("skipping user provider '%s' of unavailable type '%s'", prv.Name, prv.Type)
+			continue
 		}
 		providersMap[provider.ProviderName(prv.Name)] = p
 	}