test(providers): guard providerRegistry against AllProviderTypes drift

A type present in only one of the two hand-maintained lists fails silently:
accepted by the API then erroring "unknown provider type" at construction, or
rejected 422 despite working. Assert set-equality and canonical names so drift
fails in CI instead.

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

backend/pkg/providers/registry_test.go

@@ -0,0 +1,40 @@
+package providers
+
+import (
+	"testing"
+
+	"pentagi/pkg/providers/provider"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestProviderRegistryMatchesAllProviderTypes keeps provider.AllProviderTypes (the
+// API whitelist) and providerRegistry (construction wiring) in sync. Drift fails
+// silently: a type in only AllProviderTypes is accepted then errors "unknown
+// provider type" at construction; a type in only providerRegistry is rejected 422
+// despite working. Keep the sets equal.
+func TestProviderRegistryMatchesAllProviderTypes(t *testing.T) {
+	registryTypes := make(map[provider.ProviderType]struct{}, len(providerRegistry))
+	for _, e := range providerRegistry {
+		if _, dup := registryTypes[e.Type]; dup {
+			t.Errorf("duplicate providerRegistry entry for %q", e.Type)
+		}
+		registryTypes[e.Type] = struct{}{}
+		assert.Equal(t, provider.ProviderName(e.Type), e.Name,
+			"registry Name for %q must be the canonical provider name", e.Type)
+	}
+
+	allTypes := make(map[provider.ProviderType]struct{}, len(provider.AllProviderTypes))
+	for _, pt := range provider.AllProviderTypes {
+		allTypes[pt] = struct{}{}
+	}
+
+	for pt := range allTypes {
+		_, ok := registryTypes[pt]
+		assert.Truef(t, ok, "%q is in AllProviderTypes but missing from providerRegistry", pt)
+	}
+	for pt := range registryTypes {
+		_, ok := allTypes[pt]
+		assert.Truef(t, ok, "%q is in providerRegistry but missing from AllProviderTypes", pt)
+	}
+}