Add tests for cross-provider holding isolation and enhance fallback logic

- Introduced `Account::ProviderImportAdapterCrossProviderTest` to validate no cross-provider claiming of holdings.
- Updated `ProviderImportAdapter` to scope fallback matching by `account_provider_id`.
- Added early conflict guard and rescue for unique index violations during imports.
- Simplified rake task usage feedback.

test/models/account/provider_import_adapter_cross_provider_test.rb

@@ -0,0 +1,95 @@
+require "test_helper"
+
+# Ensures a provider cannot "claim" another provider's holdings when external_id lookup misses
+# and a (security, date, currency) match exists. The fallback path must be scoped by account_provider_id.
+class Account::ProviderImportAdapterCrossProviderTest < ActiveSupport::TestCase
+  test "does not claim holdings from a different provider when external_id is present and fallback kicks in" do
+    investment_account = accounts(:investment)
+    adapter = Account::ProviderImportAdapter.new(investment_account)
+    security = securities(:aapl)
+
+    # Create two different account providers for the SAME account
+    # Provider A (e.g., Plaid)
+    ap_a = AccountProvider.create!(
+      account: investment_account,
+      provider: plaid_accounts(:one)
+    )
+
+    # Provider B (e.g., SimpleFin)
+    item = SimplefinItem.create!(family: families(:dylan_family), name: "SF Conn", access_url: "https://example.com/access")
+    sfa_b = SimplefinAccount.create!(
+      simplefin_item: item,
+      name: "SF Invest",
+      account_id: "sf_inv_cross_provider",
+      currency: "USD",
+      account_type: "investment",
+      current_balance: 1000
+    )
+    ap_b = AccountProvider.create!(
+      account: investment_account,
+      provider: sfa_b
+    )
+
+    # Use a date that will not collide with existing fixture holdings for this account
+    holding_date = Date.today - 3.days
+
+    # Existing holding created by Provider A for (security, date, currency)
+    existing_a = investment_account.holdings.create!(
+      security: security,
+      date: holding_date,
+      qty: 1,
+      price: 100,
+      amount: 100,
+      currency: "USD",
+      account_provider_id: ap_a.id
+    )
+
+    # Now import for Provider B with an external_id that doesn't exist yet.
+    # Fallback should NOT "claim" Provider A's row because account_provider_id differs.
+    # Attempt import for Provider B with a conflicting composite key.
+    # Policy: do NOT create a duplicate row and do NOT claim Provider A's row.
+    assert_no_difference "investment_account.holdings.count" do
+      @result_b = adapter.import_holding(
+        security: security,
+        quantity: 2,
+        amount: 220,
+        currency: "USD",
+        date: holding_date,
+        price: 110,
+        cost_basis: nil,
+        external_id: "ext-b-1",
+        source: "simplefin",
+        account_provider_id: ap_b.id,
+        delete_future_holdings: false
+      )
+    end
+
+    # Provider A's holding remains unclaimed (no external_id added) and still owned by A
+    existing_a.reload
+    assert_nil existing_a.external_id
+    assert_equal ap_a.id, existing_a.account_provider_id
+
+    # Adapter returns the existing A row for transparency
+    assert_equal existing_a.id, @result_b.id
+
+    # Idempotency: importing again with the same external_id should not create another 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-b-1",
+        source: "simplefin",
+        account_provider_id: ap_b.id,
+        delete_future_holdings: false
+      )
+      assert_equal existing_a.id, again.id
+      # Ensure external_id was NOT attached to A's row (no cross-provider claim)
+      assert_nil existing_a.reload.external_id
+    end
+  end
+end