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)
+}