Add "Link to existing" option in SnapTrade Setup Accounts modal (#935)

* Add account linking functionality for SnapTrade items

- Introduced UI to link existing accounts when setting up SnapTrade items, preventing duplicate account creation.
- Updated controller to fetch linkable accounts.
- Added tests to verify proper filtering of accounts and linking behavior.

* Add `snaptrade_item_id` to account linking flow for SnapTrade items

- Updated controller to allow specifying `snaptrade_item_id` when linking accounts.
- Adjusted form and views to include `snaptrade_item_id` as a hidden field.
- Enhanced tests to validate behavior with the new parameter.

test/controllers/snaptrade_items_controller_test.rb

@@ -38,4 +38,69 @@ class SnaptradeItemsControllerTest < ActionDispatch::IntegrationTest
 
     assert_redirected_to portal_url
   end
+
+  test "setup_accounts shows linkable investment and crypto accounts in dropdown" do
+    get setup_accounts_snaptrade_item_url(@snaptrade_item)
+
+    assert_response :success
+
+    # Investment and crypto accounts (no provider) should appear in the link dropdown
+    assert_match accounts(:investment).name, response.body
+    assert_match accounts(:crypto).name, response.body
+
+    # Depository should NOT appear in the link dropdown (wrong type)
+    # The depository name may appear elsewhere on the page, so check the select options specifically
+    refute_match(/option.*#{accounts(:depository).name}/, response.body)
+  end
+
+  test "setup_accounts excludes accounts that already have a provider from dropdown" do
+    # Link the investment account to a snaptrade_account
+    AccountProvider.create!(
+      account: accounts(:investment),
+      provider: snaptrade_accounts(:fidelity_401k)
+    )
+
+    get setup_accounts_snaptrade_item_url(@snaptrade_item)
+
+    assert_response :success
+
+    # Investment account is now linked → should NOT appear in link dropdown options
+    refute_match(/option.*#{accounts(:investment).name}/, response.body)
+    # Crypto still unlinked → should appear
+    assert_match accounts(:crypto).name, response.body
+  end
+
+  test "link_existing_account links account to snaptrade_account" do
+    account = accounts(:investment)
+    snaptrade_account = snaptrade_accounts(:fidelity_401k)
+
+    assert_difference "AccountProvider.count", 1 do
+      post link_existing_account_snaptrade_items_url, params: {
+        account_id: account.id,
+        snaptrade_account_id: snaptrade_account.id,
+        snaptrade_item_id: @snaptrade_item.id
+      }
+    end
+
+    assert_redirected_to account_path(account)
+    assert_match(/Successfully linked/, flash[:notice])
+
+    snaptrade_account.reload
+    assert_equal account, snaptrade_account.current_account
+  end
+
+  test "link_existing_account handles missing account gracefully" do
+    snaptrade_account = snaptrade_accounts(:fidelity_401k)
+
+    assert_no_difference "AccountProvider.count" do
+      post link_existing_account_snaptrade_items_url, params: {
+        account_id: "nonexistent",
+        snaptrade_account_id: snaptrade_account.id,
+        snaptrade_item_id: @snaptrade_item.id
+      }
+    end
+
+    assert_redirected_to settings_providers_path
+    assert_match(/not found/i, flash[:alert])
+  end
 end