fix(trader): harden API calls with timeouts, strict balance parsing, error context

- binance/bybit/gate: SDK default http.DefaultClient has no timeout; use a
  dedicated 30s-timeout client so a hung connection cannot stall the loop
- bybit: stop mutating http.DefaultClient.Transport, which leaked the
  referer header into every other HTTP request in the process
- add types.ParseFloatField: empty exchange fields stay zero, but malformed
  numeric values now surface as errors instead of silently becoming zero
  balances (applied to GetBalance across 8 exchanges)
- wrap order/market-data errors in auto_trader_orders and okx cancel paths
  with symbol context; log per-order cancel failures in okx CancelAllOrders

trader/types/parse.go

@@ -0,0 +1,22 @@
+package types
+
+import (
+	"fmt"
+	"strconv"
+)
+
+// ParseFloatField parses a numeric string field from an exchange API response.
+// An empty string is treated as zero, since exchanges commonly omit fields
+// that have no value (e.g. unrealized PnL with no open position). A non-empty
+// value that fails to parse returns an error naming the field, so a malformed
+// API response surfaces instead of silently becoming a zero balance.
+func ParseFloatField(field, value string) (float64, error) {
+	if value == "" {
+		return 0, nil
+	}
+	v, err := strconv.ParseFloat(value, 64)
+	if err != nil {
+		return 0, fmt.Errorf("failed to parse %s value %q: %w", field, value, err)
+	}
+	return v, nil
+}

trader/types/parse_test.go

@@ -0,0 +1,44 @@
+package types
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestParseFloatField(t *testing.T) {
+	tests := []struct {
+		name    string
+		field   string
+		input   string
+		want    float64
+		wantErr bool
+	}{
+		{name: "normal value", field: "totalEq", input: "1234.56", want: 1234.56},
+		{name: "zero", field: "totalEq", input: "0", want: 0},
+		{name: "negative", field: "upl", input: "-12.5", want: -12.5},
+		{name: "empty string treated as zero", field: "upl", input: "", want: 0},
+		{name: "garbage returns error", field: "totalEq", input: "abc", wantErr: true},
+		{name: "null literal returns error", field: "totalEq", input: "null", wantErr: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := ParseFloatField(tt.field, tt.input)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatalf("ParseFloatField(%q, %q) expected error, got nil", tt.field, tt.input)
+				}
+				if !strings.Contains(err.Error(), tt.field) {
+					t.Errorf("error %q should mention field name %q", err.Error(), tt.field)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("ParseFloatField(%q, %q) unexpected error: %v", tt.field, tt.input, err)
+			}
+			if got != tt.want {
+				t.Errorf("ParseFloatField(%q, %q) = %v, want %v", tt.field, tt.input, got, tt.want)
+			}
+		})
+	}
+}