Fix SimpleFIN liability balance sign when transaction history is incomplete (#642)

* Add sanity check for net balance mismatch in overpayment analysis

- Introduced a validation to detect cases where transactions' net balance deviates significantly from the observed balance (using a 10% or $5 tolerance).
- Returns `:unknown` with a `net-balance-mismatch` error for incomplete data scenarios.

* Clarify liability net-balance comparison logic in overpayment analysis.

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

app/models/simplefin_account/liabilities/overpayment_analyzer.rb

@@ -180,6 +180,15 @@ class SimplefinAccount::Liabilities::OverpaymentAnalyzer
       end
 
       eps = [ epsilon_base, (@observed.abs * BigDecimal("0.005")) ].max
+      net = m[:charges_total] - m[:payments_total]
+
+      # Sanity check: if transaction net is way off from observed balance, data is likely incomplete
+      # (e.g., pending charges not in history yet). Use 10% tolerance or minimum $5.
+      # Note: SimpleFIN always sends negative for liabilities, so we compare magnitudes only.
+      tolerance = [ BigDecimal("5"), @observed.abs * BigDecimal("0.10") ].max
+      if (net.abs - @observed.abs).abs > tolerance
+        return [ :unknown, "net-balance-mismatch" ]
+      end
 
       # Overpayment (credit): payments exceed charges by at least the observed balance (within eps)
       if (m[:payments_total] - m[:charges_total]) >= (@observed.abs - eps)