fix: guard against nil InstrumentResolver in balance_mapper

Add nil-check at the start of ToDomainMoney and
ToDomainMoneyFromInstrumentAmount so that a missing REFERENCE_DATA_SERVICE_URL
configuration produces a controlled error rather than a request-time panic.
Introduces ErrNilInstrumentResolver sentinel for callers to distinguish this
configuration error from input validation errors.

Author: bjcoombs

services/position-keeping/adapters/balance_mapper.go

@@ -30,6 +30,9 @@ var ErrNilGoogleMoney = errors.New("MoneyAmount.Amount (google.type.Money) is ni
 // ErrInvalidCurrency is returned when the currency code is empty or invalid.
 var ErrInvalidCurrency = errors.New("invalid or empty currency code")
 
+// ErrNilInstrumentResolver is returned when a nil InstrumentResolver is provided.
+var ErrNilInstrumentResolver = errors.New("instrument resolver is required")
+
 // ErrUnknownProtoBalanceType is returned when the proto BalanceType value is not recognized.
 var ErrUnknownProtoBalanceType = errors.New("unknown proto BalanceType value")
 
@@ -118,6 +121,9 @@ func ToProtoMoneyAmount(domainMoney domain.Money) *commonv1.MoneyAmount {
 // Returns an error if the MoneyAmount or its inner google.type.Money is nil,
 // or if the instrument code cannot be resolved.
 func ToDomainMoney(ctx context.Context, resolver refdata.InstrumentResolver, protoMoney *commonv1.MoneyAmount) (domain.Money, error) {
+	if resolver == nil {
+		return domain.Money{}, ErrNilInstrumentResolver
+	}
 	if protoMoney == nil {
 		return domain.Money{}, ErrNilMoneyAmount
 	}
@@ -194,6 +200,9 @@ func ToProtoInstrumentAmountFromAsset(domainAsset domain.Asset) *quantityv1.Inst
 // registered instrument code (currencies, energy units, compute hours, carbon credits, etc.).
 // Returns an error if the amount is invalid or the instrument code cannot be resolved.
 func ToDomainMoneyFromInstrumentAmount(ctx context.Context, resolver refdata.InstrumentResolver, protoAmount *quantityv1.InstrumentAmount) (domain.Money, error) {
+	if resolver == nil {
+		return domain.Money{}, ErrNilInstrumentResolver
+	}
 	if protoAmount == nil {
 		return domain.Money{}, ErrNilInstrumentAmount
 	}

services/position-keeping/adapters/balance_mapper_test.go

@@ -324,6 +324,7 @@ func TestToDomainMoney(t *testing.T) {
 		expectedCur    domain.Currency
 		expectError    bool
 		errorIs        error
+		nilResolver    bool
 	}{
 		{
 			name: "GBP with cents",
@@ -403,6 +404,13 @@ func TestToDomainMoney(t *testing.T) {
 			expectedCur:    domain.CurrencyCHF,
 			expectError:    false,
 		},
+		{
+			name:        "nil resolver returns error",
+			proto:       &commonv1.MoneyAmount{Amount: &money.Money{CurrencyCode: "GBP"}},
+			expectError: true,
+			errorIs:     adapters.ErrNilInstrumentResolver,
+			nilResolver: true,
+		},
 		{
 			name:        "nil MoneyAmount returns error",
 			proto:       nil,
@@ -445,7 +453,11 @@ func TestToDomainMoney(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			result, err := adapters.ToDomainMoney(ctx, resolver, tt.proto)
+			r := refdata.InstrumentResolver(resolver)
+			if tt.nilResolver {
+				r = nil
+			}
+			result, err := adapters.ToDomainMoney(ctx, r, tt.proto)
 
 			if tt.expectError {
 				require.Error(t, err)
@@ -717,7 +729,19 @@ func TestToDomainMoneyFromInstrumentAmount(t *testing.T) {
 		errContains  string
 		expectAmount string
 		expectCode   string
+		nilResolver  bool
 	}{
+		{
+			name: "nil resolver returns error",
+			proto: &quantityv1.InstrumentAmount{
+				Amount:         "100",
+				InstrumentCode: "GBP",
+				Version:        1,
+			},
+			expectError: true,
+			errContains: "resolver",
+			nilResolver: true,
+		},
 		{
 			name: "valid GBP amount",
 			proto: &quantityv1.InstrumentAmount{
@@ -823,7 +847,11 @@ func TestToDomainMoneyFromInstrumentAmount(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			result, err := adapters.ToDomainMoneyFromInstrumentAmount(ctx, resolver, tt.proto)
+			r := refdata.InstrumentResolver(resolver)
+			if tt.nilResolver {
+				r = nil
+			}
+			result, err := adapters.ToDomainMoneyFromInstrumentAmount(ctx, r, tt.proto)
 
 			if tt.expectError {
 				require.Error(t, err)