fix: skip auth requirement for config-defined custom providers

Shows the auth panel only when provider credentials are actually required
Keeps custom providers defined in config from being treated as incomplete
Adds coverage for the new provider auth check

packages/ui/src/components/sections/providers/ProvidersPage.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, test } from 'bun:test';
-import { shouldLoadAvailableProviders } from './providerAvailability';
+import { requiresProviderAuth, shouldLoadAvailableProviders } from './providerAvailability';
 import {
   getOAuthAuthMethods,
   normalizeAuthType,
@@ -15,6 +15,14 @@ describe('ProvidersPage available provider loading', () => {
   });
 });
 
+describe('ProvidersPage provider authentication', () => {
+  test('does not require credentials for a custom provider defined in config', () => {
+    expect(requiresProviderAuth(true, false, true)).toBe(false);
+    expect(requiresProviderAuth(true, false, false)).toBe(true);
+    expect(requiresProviderAuth(true, true, false)).toBe(false);
+  });
+});
+
 describe('provider auth method helpers', () => {
   test('normalizeAuthType recognizes oauth and api labels', () => {
     expect(normalizeAuthType({ type: 'oauth', label: 'Login with Cursor' })).toBe('oauth');