Add tests and logic for adopting unowned holdings during provider imports

- Introduced `Account::ProviderImportAdapterUnownedAdoptionTest` to verify adoption of unowned holdings on collision.
- Updated `ProviderImportAdapter` to claim unowned holdings by updating attributes, attaching external_id, and setting account_provider_id.
- Ensured idempotency in subsequent imports for the same unowned holding.

test/models/account/provider_import_adapter_unowned_adoption_test.rb

@@ -0,0 +1,90 @@
+require "test_helper"
+
+# Ensures an "unowned" composite row (no account_provider_id) is fully adopted on
+# first collision: attributes are updated, external_id attached, and
+# account_provider_id set to the importing provider.
+class Account::ProviderImportAdapterUnownedAdoptionTest < ActiveSupport::TestCase
+  test "adopts unowned holding on unique-index collision by updating attrs and provider ownership" do
+    investment_account = accounts(:investment)
+    adapter = Account::ProviderImportAdapter.new(investment_account)
+    security = securities(:aapl)
+
+    # Create a SimpleFin provider for this account (the importer)
+    item = SimplefinItem.create!(family: families(:dylan_family), name: "SF Conn", access_url: "https://example.com/access")
+    sfa = SimplefinAccount.create!(
+      simplefin_item: item,
+      name: "SF Invest",
+      account_id: "sf_inv_unowned_claim",
+      currency: "USD",
+      account_type: "investment",
+      current_balance: 1000
+    )
+    ap = AccountProvider.create!(account: investment_account, provider: sfa)
+
+    holding_date = Date.today - 4.days
+
+    # Existing composite row without provider ownership (unowned)
+    existing_unowned = investment_account.holdings.create!(
+      security: security,
+      date: holding_date,
+      qty: 1,
+      price: 100,
+      amount: 100,
+      currency: "USD",
+      account_provider_id: nil
+    )
+
+    # Import for SimpleFin with an external_id that will collide on composite key
+    # Adapter should NOT create a new row, but should update the existing one:
+    # - qty/price/amount/cost_basis updated
+    # - external_id attached
+    # - account_provider_id adopted to ap.id
+    assert_no_difference "investment_account.holdings.count" do
+      @result = adapter.import_holding(
+        security: security,
+        quantity: 2,
+        amount: 220,
+        currency: "USD",
+        date: holding_date,
+        price: 110,
+        cost_basis: nil,
+        external_id: "ext-unowned-1",
+        source: "simplefin",
+        account_provider_id: ap.id,
+        delete_future_holdings: false
+      )
+    end
+
+    existing_unowned.reload
+
+    # Attributes updated
+    assert_equal 2, existing_unowned.qty
+    assert_equal 110, existing_unowned.price
+    assert_equal 220, existing_unowned.amount
+
+    # Ownership and external_id adopted
+    assert_equal ap.id, existing_unowned.account_provider_id
+    assert_equal "ext-unowned-1", existing_unowned.external_id
+
+    # Adapter returns the same row
+    assert_equal existing_unowned.id, @result.id
+
+    # Idempotency: re-import should not create a duplicate and should return the same row
+    assert_no_difference "investment_account.holdings.count" do
+      again = adapter.import_holding(
+        security: security,
+        quantity: 2,
+        amount: 220,
+        currency: "USD",
+        date: holding_date,
+        price: 110,
+        cost_basis: nil,
+        external_id: "ext-unowned-1",
+        source: "simplefin",
+        account_provider_id: ap.id,
+        delete_future_holdings: false
+      )
+      assert_equal existing_unowned.id, again.id
+    end
+  end
+end