Eliminate code duplication in OIDC identity creation (#230)

* Eliminate duplication by using create_from_omniauth method

- Updated OidcIdentity.create_from_omniauth to set last_authenticated_at
- Refactored OidcAccountsController to use create_from_omniauth instead of direct create! calls
- Updated test to verify last_authenticated_at is set by create_from_omniauth

Co-authored-by: jjmata <187772+jjmata@users.noreply.github.com>

* Extract auth hash building into private helper method

- Added build_auth_hash helper method to eliminate OpenStruct creation duplication
- Both create_link and create_user actions now use the same helper

Co-authored-by: jjmata <187772+jjmata@users.noreply.github.com>

* Linter fix

* Fix button style on OIDC link step

* Fix dark mode styles

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jjmata <187772+jjmata@users.noreply.github.com>
Co-authored-by: Juan José Mata <juanjo.mata@gmail.com>

app/views/oidc_accounts/link.html.erb

@@ -42,21 +42,33 @@
   </div>
 
   <div class="space-y-4">
-    <div class="p-4 bg-gray-50 border border-gray-200 rounded-md">
-      <p class="text-sm text-gray-700">
+    <div class="p-4 bg-container border border-secondary rounded-md">
+      <p class="text-sm text-primary">
         <strong>Email:</strong> <%= @pending_auth["email"] %>
       </p>
       <% if @pending_auth["name"].present? %>
-        <p class="text-sm text-gray-700 mt-2">
+        <p class="text-sm text-primary mt-2">
           <strong>Name:</strong> <%= @pending_auth["name"] %>
         </p>
       <% end %>
     </div>
 
-    <%= button_to "Create Account", create_user_oidc_account_path, method: :post, class: "w-full", data: { turbo: false } %>
+    <%= render DS::Button.new(
+      text: "Create Account",
+      href: create_user_oidc_account_path,
+      full_width: true,
+      variant: :primary,
+      method: :post,
+      data: { turbo: false }
+    ) %>
   </div>
 <% end %>
 
 <div class="mt-6 text-center">
-  <%= link_to "Cancel", new_session_path, class: "font-medium text-sm text-primary hover:underline transition" %>
+  <%= render DS::Link.new(
+    text: "Cancel",
+    href: new_session_path,
+    variant: :default,
+    class: "font-medium text-sm text-primary hover:underline transition"
+  ) %>
 </div>