Add overpayment detection for SimpleFIN liabilities (default ON) with heuristic-based classification and robust fallbacks (#412)

* Add liability balance normalization logic with comprehensive tests

- Updated `SimplefinAccount::Processor` to normalize liability balances based on observed values, ensuring correct handling of debts and overpayments.
- Enhanced `SimplefinItem::Importer` to apply similar normalization rules during imports, improving consistency.
- Added multiple test cases in `SimplefinAccountProcessorTest` to validate edge cases for liabilities and mixed-sign scenarios.
- Introduced helper methods (`to_decimal`, `same_sign?`) to simplify numeric operations in normalization logic.

* Add overpayment detection for liabilities with heuristic-based classification

- Introduced `SimplefinAccount::Liabilities::OverpaymentAnalyzer` to classify liability balances as credit, debt, or unknown using transaction history.
- Updated `SimplefinAccount::Processor` and `SimplefinItem::Importer` to integrate heuristic-based balance normalization with fallback logic for ambiguous cases.
- Added comprehensive unit tests in `OverpaymentAnalyzerTest` to validate classification logic and edge cases.
- Enhanced logging and observability around classification results and fallback scenarios.

* Refactor liability handling for better fallback consistency

- Updated `sticky_key` method in `OverpaymentAnalyzer` to handle missing `@sfa.id` with a default value.
- Enhanced `SimplefinAccount::Processor` to use `with_indifferent_access` for `raw_payload` and `org_data`, improving robustness in liability type inference.

* Extract numeric helper methods into `SimplefinNumericHelpers` concern and apply across models

- Moved `to_decimal` and `same_sign?` methods into a new `SimplefinNumericHelpers` concern for reuse.
- Updated `OverpaymentAnalyzer`, `Processor`, and `Importer` to include the concern and remove redundant method definitions.
- Added empty fixtures for `simplefin_accounts` and `simplefin_items` to ensure test isolation.
- Refactored `OverpaymentAnalyzerTest` to reduce fixture dependencies and ensure cleanup of created records.

* Refactor overpayment detection logic for clarity and fallback consistency

- Simplified `enabled?` method in `OverpaymentAnalyzer` for clearer precedence order (Setting > ENV > default).
- Added `parse_bool` helper to streamline boolean parsing.
- Enhanced error handling with detailed logging for transaction gathering failures.
- Improved `sticky_key` method to use a temporary object ID fallback when `@sfa.id` is missing.

---------

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

test/models/simplefin_account_processor_test.rb

@@ -81,4 +81,101 @@ class SimplefinAccountProcessorTest < ActiveSupport::TestCase
 
     assert_equal BigDecimal("-75.00"), acct.reload.balance
   end
+
+  test "liability debt with both fields negative becomes positive (you owe)" do
+    sfin_acct = SimplefinAccount.create!(
+      simplefin_item: @item,
+      name: "BofA Visa",
+      account_id: "cc_bofa_1",
+      currency: "USD",
+      account_type: "credit",
+      current_balance: BigDecimal("-1200"),
+      available_balance: BigDecimal("-5000")
+    )
+
+    acct = accounts(:credit_card)
+    acct.update!(simplefin_account: sfin_acct)
+
+    SimplefinAccount::Processor.new(sfin_acct).send(:process_account!)
+
+    assert_equal BigDecimal("1200"), acct.reload.balance
+  end
+
+  test "liability overpayment with both fields positive becomes negative (credit)" do
+    sfin_acct = SimplefinAccount.create!(
+      simplefin_item: @item,
+      name: "BofA Visa",
+      account_id: "cc_bofa_2",
+      currency: "USD",
+      account_type: "credit",
+      current_balance: BigDecimal("75"),
+      available_balance: BigDecimal("5000")
+    )
+
+    acct = accounts(:credit_card)
+    acct.update!(simplefin_account: sfin_acct)
+
+    SimplefinAccount::Processor.new(sfin_acct).send(:process_account!)
+
+    assert_equal BigDecimal("-75"), acct.reload.balance
+  end
+
+  test "mixed signs falls back to invert observed (balance positive, avail negative => negative)" do
+    sfin_acct = SimplefinAccount.create!(
+      simplefin_item: @item,
+      name: "Chase Freedom",
+      account_id: "cc_chase_1",
+      currency: "USD",
+      account_type: "credit",
+      current_balance: BigDecimal("50"),
+      available_balance: BigDecimal("-5000")
+    )
+
+    acct = accounts(:credit_card)
+    acct.update!(simplefin_account: sfin_acct)
+
+    SimplefinAccount::Processor.new(sfin_acct).send(:process_account!)
+
+    assert_equal BigDecimal("-50"), acct.reload.balance
+  end
+
+  test "only available-balance present positive → negative (credit) for liability" do
+    sfin_acct = SimplefinAccount.create!(
+      simplefin_item: @item,
+      name: "Chase Visa",
+      account_id: "cc_chase_2",
+      currency: "USD",
+      account_type: "credit",
+      current_balance: nil,
+      available_balance: BigDecimal("25")
+    )
+
+    acct = accounts(:credit_card)
+    acct.update!(simplefin_account: sfin_acct)
+
+    SimplefinAccount::Processor.new(sfin_acct).send(:process_account!)
+
+    assert_equal BigDecimal("-25"), acct.reload.balance
+  end
+
+  test "mislinked as asset but mapper infers credit → normalize as liability" do
+    sfin_acct = SimplefinAccount.create!(
+      simplefin_item: @item,
+      name: "Visa Signature",
+      account_id: "cc_mislinked",
+      currency: "USD",
+      account_type: "credit",
+      current_balance: BigDecimal("100.00"),
+      available_balance: BigDecimal("5000.00")
+    )
+
+    # Link to an asset account intentionally
+    acct = accounts(:depository)
+    acct.update!(simplefin_account: sfin_acct)
+
+    SimplefinAccount::Processor.new(sfin_acct).send(:process_account!)
+
+    # Mapper should infer liability from name; final should be negative
+    assert_equal BigDecimal("-100.00"), acct.reload.balance
+  end
 end