Simplefin liabilities recording fix (#410)

* Add tests and logic for Simplefin account balance normalization

- Introduced `SimplefinAccountProcessorTest` to verify balance normalization logic.
- Updated `SimplefinAccount::Processor` to invert negative balances for liability accounts (credit cards and loans) while keeping asset balances unchanged.
- Added comments to clarify balance conventions and sign normalization rules.

* Refactor balances-only sync logic and improve tests for edge cases

- Updated `SimplefinItem::Importer` and `SimplefinItem::Syncer` to ensure `last_synced_at` remains nil during balances-only runs, preserving chunked-history behavior for full syncs.
- Introduced additional comments to clarify balances-only implications and syncing logic.
- Added test case in `SimplefinAccountProcessorTest` to verify correct handling of overpayment for credit card liabilities.
- Refined balance normalization in `SimplefinAccount::Processor` to always invert liability balances for consistency.

---------

Co-authored-by: Josh Waldrep <joshua.waldrep5+github@gmail.com>

test/models/simplefin_account_processor_test.rb

@@ -0,0 +1,84 @@
+require "test_helper"
+
+class SimplefinAccountProcessorTest < ActiveSupport::TestCase
+  setup do
+    @family = families(:dylan_family)
+    @item = SimplefinItem.create!(
+      family: @family,
+      name: "SimpleFIN",
+      access_url: "https://example.com/token"
+    )
+  end
+
+  test "inverts negative balance for credit card liabilities" do
+    sfin_acct = SimplefinAccount.create!(
+      simplefin_item: @item,
+      name: "Chase Credit",
+      account_id: "cc_1",
+      currency: "USD",
+      account_type: "credit",
+      current_balance: BigDecimal("-123.45")
+    )
+
+    acct = accounts(:credit_card)
+    acct.update!(simplefin_account: sfin_acct)
+
+    SimplefinAccount::Processor.new(sfin_acct).send(:process_account!)
+
+    assert_equal BigDecimal("123.45"), acct.reload.balance
+  end
+
+  test "does not invert balance for asset accounts (depository)" do
+    sfin_acct = SimplefinAccount.create!(
+      simplefin_item: @item,
+      name: "Checking",
+      account_id: "dep_1",
+      currency: "USD",
+      account_type: "checking",
+      current_balance: BigDecimal("1000.00")
+    )
+
+    acct = accounts(:depository)
+    acct.update!(simplefin_account: sfin_acct)
+
+    SimplefinAccount::Processor.new(sfin_acct).send(:process_account!)
+
+    assert_equal BigDecimal("1000.00"), acct.reload.balance
+  end
+
+  test "inverts negative balance for loan liabilities" do
+    sfin_acct = SimplefinAccount.create!(
+      simplefin_item: @item,
+      name: "Mortgage",
+      account_id: "loan_1",
+      currency: "USD",
+      account_type: "mortgage",
+      current_balance: BigDecimal("-50000")
+    )
+
+    acct = accounts(:loan)
+    acct.update!(simplefin_account: sfin_acct)
+
+    SimplefinAccount::Processor.new(sfin_acct).send(:process_account!)
+
+    assert_equal BigDecimal("50000"), acct.reload.balance
+  end
+
+  test "positive provider balance (overpayment) becomes negative for credit card liabilities" do
+    sfin_acct = SimplefinAccount.create!(
+      simplefin_item: @item,
+      name: "Chase Credit",
+      account_id: "cc_overpay",
+      currency: "USD",
+      account_type: "credit",
+      current_balance: BigDecimal("75.00") # provider sends positive for overpayment
+    )
+
+    acct = accounts(:credit_card)
+    acct.update!(simplefin_account: sfin_acct)
+
+    SimplefinAccount::Processor.new(sfin_acct).send(:process_account!)
+
+    assert_equal BigDecimal("-75.00"), acct.reload.balance
+  end
+end