feat: ErrorPolicySkipped returns 500 instead of upstream status code

When custom error codes are enabled and the upstream error code is NOT
in the configured list, return HTTP 500 to the client instead of
transparently forwarding the original status code. This matches the
frontend description: "other errors will return 500".

Also adds integration test TestCustomErrorCode599 verifying that 429,
500, 503, 401, 403 all return 500 without triggering SetRateLimited
or SetError.

backend/internal/service/error_policy_test.go

@@ -158,6 +158,7 @@ func TestApplyErrorPolicy(t *testing.T) {
 		statusCode        int
 		body              []byte
 		expectedHandled   bool
+		expectedStatus    int  // expected outStatus
 		expectedSwitchErr bool // expect *AntigravityAccountSwitchError
 		handleErrorCalls  int
 	}{
@@ -171,6 +172,7 @@ func TestApplyErrorPolicy(t *testing.T) {
 			statusCode:       500,
 			body:             []byte(`"error"`),
 			expectedHandled:  false,
+			expectedStatus:   500, // passthrough
 			handleErrorCalls: 0,
 		},
 		{
@@ -187,6 +189,7 @@ func TestApplyErrorPolicy(t *testing.T) {
 			statusCode:       500, // not in custom codes
 			body:             []byte(`"error"`),
 			expectedHandled:  true,
+			expectedStatus:   http.StatusInternalServerError, // skipped → 500
 			handleErrorCalls: 0,
 		},
 		{
@@ -203,6 +206,7 @@ func TestApplyErrorPolicy(t *testing.T) {
 			statusCode:       500,
 			body:             []byte(`"error"`),
 			expectedHandled:  true,
+			expectedStatus:   500, // matched → original status
 			handleErrorCalls: 1,
 		},
 		{
@@ -225,6 +229,7 @@ func TestApplyErrorPolicy(t *testing.T) {
 			statusCode:        503,
 			body:              []byte(`overloaded`),
 			expectedHandled:   true,
+			expectedStatus:    503, // temp_unscheduled → original status
 			expectedSwitchErr: true,
 			handleErrorCalls:  0,
 		},
@@ -250,9 +255,10 @@ func TestApplyErrorPolicy(t *testing.T) {
 				isStickySession: true,
 			}
 
-			handled, retErr := svc.applyErrorPolicy(p, tt.statusCode, http.Header{}, tt.body)
+			handled, outStatus, retErr := svc.applyErrorPolicy(p, tt.statusCode, http.Header{}, tt.body)
 
 			require.Equal(t, tt.expectedHandled, handled, "handled mismatch")
+			require.Equal(t, tt.expectedStatus, outStatus, "outStatus mismatch")
 			require.Equal(t, tt.handleErrorCalls, handleErrorCount, "handleError call count mismatch")
 
 			if tt.expectedSwitchErr {