Add tests for SnapTrade error handling and refine unlink behavior (#931)

- Introduced new tests to cover SnapTrade decryption and connection errors in `SnaptradeItemsControllerTest`.
- Updated error messages for improved user clarity.
- Modified `unlink` functionality to preserve `SnaptradeAccount` records while ensuring proper detachment of associated holdings.

test/controllers/snaptrade_items_controller_test.rb

@@ -0,0 +1,41 @@
+require "test_helper"
+
+class SnaptradeItemsControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    sign_in @user = users(:family_admin)
+    @snaptrade_item = snaptrade_items(:configured_item)
+  end
+
+  test "connect handles decryption error gracefully" do
+    SnaptradeItem.any_instance
+      .stubs(:user_registered?)
+      .raises(ActiveRecord::Encryption::Errors::Decryption.new("cannot decrypt"))
+
+    get connect_snaptrade_item_url(@snaptrade_item)
+
+    assert_redirected_to settings_providers_path
+    assert_match(/Unable to read SnapTrade credentials/, flash[:alert])
+  end
+
+  test "connect handles general error gracefully" do
+    SnaptradeItem.any_instance
+      .stubs(:user_registered?)
+      .raises(StandardError.new("something broke"))
+
+    get connect_snaptrade_item_url(@snaptrade_item)
+
+    assert_redirected_to settings_providers_path
+    assert_match(/Failed to connect/, flash[:alert])
+  end
+
+  test "connect redirects to portal when successful" do
+    portal_url = "https://app.snaptrade.com/portal/test123"
+
+    SnaptradeItem.any_instance.stubs(:user_registered?).returns(true)
+    SnaptradeItem.any_instance.stubs(:connection_portal_url).returns(portal_url)
+
+    get connect_snaptrade_item_url(@snaptrade_item)
+
+    assert_redirected_to portal_url
+  end
+end