fix: strip Claude Code attribution from non-Anthropic translations

internal/util/claude_attribution.go

@@ -0,0 +1,15 @@
+package util
+
+import (
+	"strings"
+	"unicode"
+)
+
+const claudeCodeAttributionSystemPrefix = "x-anthropic-billing-header:"
+
+// IsClaudeCodeAttributionSystemText reports whether text is the Claude Code
+// attribution block that carries per-request billing and prompt fingerprint data.
+func IsClaudeCodeAttributionSystemText(text string) bool {
+	text = strings.TrimLeftFunc(text, unicode.IsSpace)
+	return strings.HasPrefix(text, claudeCodeAttributionSystemPrefix)
+}

internal/util/claude_attribution_test.go

@@ -0,0 +1,40 @@
+package util
+
+import "testing"
+
+func TestIsClaudeCodeAttributionSystemText(t *testing.T) {
+	tests := []struct {
+		name string
+		text string
+		want bool
+	}{
+		{
+			name: "Claude Code attribution block",
+			text: "x-anthropic-billing-header: cc_version=2.1.63.abc; cc_entrypoint=cli; cch=12345;",
+			want: true,
+		},
+		{
+			name: "leading whitespace",
+			text: "\n\t x-anthropic-billing-header: cc_version=2.1.63.abc; cch=12345;",
+			want: true,
+		},
+		{
+			name: "regular system prompt",
+			text: "You are helpful.",
+			want: false,
+		},
+		{
+			name: "empty text",
+			text: "",
+			want: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := IsClaudeCodeAttributionSystemText(tt.text); got != tt.want {
+				t.Fatalf("IsClaudeCodeAttributionSystemText(%q) = %v, want %v", tt.text, got, tt.want)
+			}
+		})
+	}
+}