Fix avg_cost to return per-share cost basis (#1692)

* Fix avg_cost to return per-share cost basis

* Revert "Fix avg_cost to return per-share cost basis"

This reverts commit 38c438a614.

* Normalize SimpleFIN holding cost basis

* Track SimpleFIN cost basis source field

* Add SimpleFIN cost basis normalization tests

* Update SimpleFIN value cost basis expectation

* Handle missing SimpleFIN quantity for cost basis

* Ignore missing SimpleFIN cost basis fields

* Fix SimpleFIN holdings processor test setup

app/models/simplefin_account/investments/holdings_processor.rb

@@ -47,7 +47,8 @@ class SimplefinAccount::Investments::HoldingsProcessor
         # which would cause the system to display average cost as current price. (GH #1182)
         qty = parse_decimal(any_of(simplefin_holding, %w[shares quantity qty units]))
         market_value = parse_decimal(any_of(simplefin_holding, %w[market_value current_value]))
-        cost_basis = parse_decimal(any_of(simplefin_holding, %w[cost_basis basis total_cost value]))
+        raw_cost_basis, cost_basis_source_key = cost_basis_from(simplefin_holding)
+        cost_basis = normalize_cost_basis(raw_cost_basis, qty, cost_basis_source_key)
 
         # Derive price from market_value when possible; otherwise fall back to any price field
         fallback_price = parse_decimal(any_of(simplefin_holding, %w[purchase_price price unit_price average_cost avg_cost]))
@@ -112,6 +113,32 @@ class SimplefinAccount::Investments::HoldingsProcessor
       simplefin_account.raw_holdings_payload || []
     end
 
+    def cost_basis_from(simplefin_holding)
+      %w[cost_basis basis total_cost value].each do |key|
+        raw = simplefin_holding[key]
+        next if raw.nil? || raw.to_s.strip.empty?
+
+        return [ parse_decimal(raw), key ]
+      end
+
+      [ nil, nil ]
+    end
+
+    # Sure stores holding cost_basis as per-share average cost. Some SimpleFIN
+    # providers expose total position basis via total_cost/value, so normalize only
+    # when the selected provider field is known to represent total position basis.
+    def normalize_cost_basis(raw_cost_basis, qty, source_key)
+      return nil if raw_cost_basis.nil?
+
+      if %w[total_cost value].include?(source_key)
+        return nil unless qty.to_d.positive?
+
+        raw_cost_basis / qty
+      else
+        raw_cost_basis
+      end
+    end
+
     def resolve_security(symbol, description)
       # Normalize crypto tickers to a distinct namespace so they don't collide with equities
       sym = symbol.to_s.upcase