Fix income trades (interest/dividend) wiping security holdings to $0 (#2673)

Income trades created via Trade::CreateForm#create_income_trade set
price: 0 on the Trade record. This zero price was ingested by
PortfolioCache#load_prices as a valid price source, causing
ForwardCalculator#build_holdings to compute holding amount = qty * 0 = $0,
and overriding the security's market price on that date.

Additionally, Balance::BaseCalculator#flows_for_date misclassified income
trades (qty=0) as sells, producing spurious non_cash_outflows.

Fixes:
- PortfolioCache#load_prices: filter out zero-price trades from trade
  price sources using .select { |t| t.entryable.price&.positive? }
- Balance::BaseCalculator#flows_for_date: separate income trades from
  regular trades using their qty (income trades have qty=0), so they
  contribute only to cash flows, not non-cash flows
- Balance::SyncCache#converted_entries: preserve the entryable association
  target on duped entries so that e.entryable.qty access doesn't N+1

Fixes #2672

test/support/ledger_testing_helper.rb

@@ -88,6 +88,29 @@ module LedgerTestingHelper
           currency: currency,
           entryable: trade
         )
+      when "income_trade"
+        security = Security.find_or_create_by!(ticker: entry_data[:ticker]) do |s|
+          s.name = entry_data[:ticker]
+        end
+
+        currency = entry_data[:currency] || created_account.currency
+
+        trade = Trade.new(
+          qty: 0,
+          security: security,
+          price: 0,
+          fee: 0,
+          currency: currency,
+          investment_activity_label: entry_data[:label] || "Interest"
+        )
+
+        created_account.entries.create!(
+          name: "#{entry_data[:label] || 'Interest'}: #{entry_data[:ticker]}",
+          date: entry_data[:date],
+          amount: -(entry_data[:amount].to_d),
+          currency: currency,
+          entryable: trade
+        )
       end
     end