Add support to unlink lunch flow accounts (#318)

* Add support to unlink lunch flow accounts

* add support to link and unlink to any provider

* Fix tests and query

* Let's keep Amr happy about his brand

* Wrap unlink operations in a transaction and add error handling.

* Fix tests

---------

Co-authored-by: Juan José Mata <juanjo.mata@gmail.com>

test/controllers/plaid_items_controller_test.rb

@@ -46,4 +46,45 @@ class PlaidItemsControllerTest < ActionDispatch::IntegrationTest
 
     assert_redirected_to accounts_path
   end
+
+  test "select_existing_account redirects when no available plaid accounts" do
+    account = accounts(:depository)
+
+    get select_existing_account_plaid_items_url(account_id: account.id, region: "us")
+    assert_redirected_to account_path(account)
+    assert_equal "No available Plaid accounts to link. Please connect a new Plaid account first.", flash[:alert]
+  end
+
+  test "link_existing_account links plaid account to existing account" do
+    account = accounts(:depository)
+
+    # Create a new unlinked plaid_account for testing
+    plaid_account = PlaidAccount.create!(
+      plaid_item: plaid_items(:one),
+      name: "Test Plaid Account",
+      plaid_id: "test_acc_123",
+      plaid_type: "depository",
+      plaid_subtype: "checking",
+      currency: "USD",
+      current_balance: 1000,
+      available_balance: 1000
+    )
+
+    assert_not account.linked?
+    assert_nil plaid_account.account
+    assert_nil plaid_account.account_provider
+
+    assert_difference "AccountProvider.count", 1 do
+      post link_existing_account_plaid_items_url, params: {
+        account_id: account.id,
+        plaid_account_id: plaid_account.id
+      }
+    end
+
+    account.reload
+    assert account.linked?, "Account should be linked after creating AccountProvider"
+    assert_equal 1, account.account_providers.count
+    assert_redirected_to accounts_path
+    assert_equal "Account successfully linked to Plaid", flash[:notice]
+  end
 end