fix(ledger): split dividend response schema and harden decimal tests

Co-authored-by: genichin <4282227+genichin@users.noreply.github.com>

tests/test_v1_portfolios_manual_ledger.py

@@ -981,9 +981,12 @@ def test_unified_ledger_openapi_contains_nullable_amount() -> None:
     response = client.get("/openapi.json")
     assert response.status_code == 200
     schemas = response.json()["components"]["schemas"]
-    assert "amount" in schemas["LedgerHistoryItem"]["properties"]
-    amount_schema = schemas["ManualLedgerResponse"]["properties"]["amount"]
-    assert any(option.get("type") == "number" for option in amount_schema.get("anyOf", []))
+    manual_amount_types = {option.get("type") for option in schemas["ManualLedgerResponse"]["properties"]["amount"]["anyOf"]}
+    assert manual_amount_types == {"number", "null"}
+    assert schemas["DividendManualLedgerResponse"]["properties"]["amount"]["type"] == "string"
+    history_amount_types = {option.get("type") for option in schemas["LedgerHistoryItem"]["properties"]["amount"]["anyOf"]}
+    assert history_amount_types == {"number", "null"}
+    assert schemas["DividendLedgerHistoryItem"]["properties"]["amount"]["type"] == "string"
 
 
 def test_reversal_rejects_fee_tax_and_missing_target() -> None: