fix: restrict existing user sign-in to account members (#13793)

SAML sign-in now only links an existing user when that user already
belongs to the account that initiated SSO. New users can still be
created for SAML-enabled accounts, and invited members can continue to
sign in through their IdP, but SAML will no longer auto-attach an
unrelated existing user record during login.

**What changed**
- Added an account-membership check before SAML reuses an existing user
by email.
- Kept first-time SAML user creation unchanged for valid new users.
- Added builder and request specs covering the allowed and rejected
login paths.

enterprise/app/controllers/enterprise/devise_overrides/omniauth_callbacks_controller.rb

@@ -39,7 +39,7 @@ module Enterprise::DeviseOverrides::OmniauthCallbacksController
     error = params[:message] || 'authentication-failed'
 
     if for_mobile?(relay_state)
-      redirect_to_mobile_error(error, relay_state)
+      redirect_to_mobile_error(error)
     else
       redirect_to login_page_url(error: "saml-#{error}")
     end
@@ -51,23 +51,15 @@ module Enterprise::DeviseOverrides::OmniauthCallbacksController
     account_id = extract_saml_account_id
     relay_state = saml_relay_state
 
-    unless saml_enabled_for_account?(account_id)
-      return redirect_to_mobile_error('saml-not-enabled') if for_mobile?(relay_state)
-
-      return redirect_to login_page_url(error: 'saml-not-enabled')
-    end
+    return handle_saml_auth_error(relay_state, 'saml-not-enabled') unless saml_enabled_for_account?(account_id)
 
     @resource = SamlUserBuilder.new(auth_hash, account_id).perform
 
-    if @resource.persisted?
-      return sign_in_user_on_mobile if for_mobile?(relay_state)
+    return sign_in_saml_user(relay_state) if @resource.persisted?
 
-      sign_in_user
-    else
-      return redirect_to_mobile_error('saml-authentication-failed') if for_mobile?(relay_state)
-
-      redirect_to login_page_url(error: 'saml-authentication-failed')
-    end
+    handle_saml_auth_error(relay_state, 'saml-authentication-failed')
+  rescue SamlUserBuilder::AuthenticationFailed
+    handle_saml_auth_error(relay_state, 'saml-authentication-failed')
   end
 
   def extract_saml_account_id
@@ -82,6 +74,18 @@ module Enterprise::DeviseOverrides::OmniauthCallbacksController
     relay_state.to_s.casecmp('mobile').zero?
   end
 
+  def sign_in_saml_user(relay_state)
+    return sign_in_user_on_mobile if for_mobile?(relay_state)
+
+    sign_in_user
+  end
+
+  def handle_saml_auth_error(relay_state, error)
+    return redirect_to_mobile_error(error) if for_mobile?(relay_state)
+
+    redirect_to login_page_url(error: error)
+  end
+
   def redirect_to_mobile_error(error)
     mobile_deep_link_base = GlobalConfigService.load('MOBILE_DEEP_LINK_BASE', 'chatwootapp')
     redirect_to "#{mobile_deep_link_base}://auth/saml?error=#{ERB::Util.url_encode(error)}", allow_other_host: true