Fix broken Git over HTTP clones with LDAP users

Due to a regression in !20608, the LDAP authenticator was not being used
unless OmniAuth was enabled. This change allows the LDAP provider to be used
if it is configured regardless of the OmniAuth setting.

Closes #50579

changelogs/unreleased/sh-fix-broken-ldap-clones.yml

+---
+title: Fix broken Git over HTTP clones with LDAP users
+merge_request: 21352
+author:
+type: fixed

lib/gitlab/auth/o_auth/provider.rb

@@ -29,6 +29,7 @@ module Gitlab
 
         def self.enabled?(name)
           return true if name == 'database'
+          return true if self.ldap_provider?(name) && providers.include?(name.to_sym)
 
           Gitlab::Auth.omniauth_enabled? && providers.include?(name.to_sym)
         end

spec/lib/gitlab/auth/o_auth/provider_spec.rb

 require 'spec_helper'
 
 describe Gitlab::Auth::OAuth::Provider do
+  describe '.enabled?' do
+    before do
+      allow(described_class).to receive(:providers).and_return([:ldapmain, :google_oauth2])
+    end
+
+    context 'when OmniAuth is disabled' do
+      before do
+        allow(Gitlab::Auth).to receive(:omniauth_enabled?).and_return(false)
+      end
+
+      it 'allows database auth' do
+        expect(described_class.enabled?('database')).to be_truthy
+      end
+
+      it 'allows LDAP auth' do
+        expect(described_class.enabled?('ldapmain')).to be_truthy
+      end
+
+      it 'does not allow other OmniAuth providers' do
+        expect(described_class.enabled?('google_oauth2')).to be_falsey
+      end
+    end
+
+    context 'when OmniAuth is enabled' do
+      before do
+        allow(Gitlab::Auth).to receive(:omniauth_enabled?).and_return(true)
+      end
+
+      it 'allows database auth' do
+        expect(described_class.enabled?('database')).to be_truthy
+      end
+
+      it 'allows LDAP auth' do
+        expect(described_class.enabled?('ldapmain')).to be_truthy
+      end
+
+      it 'allows other OmniAuth providers' do
+        expect(described_class.enabled?('google_oauth2')).to be_truthy
+      end
+    end
+  end
+
   describe '#config_for' do
     context 'for an LDAP provider' do
       context 'when the provider exists' do