fix: filter Anthropic reserved keywords from system prompts

Filter out Anthropic-specific headers like x-anthropic-billing-header
and x-anthropic-billing from system prompts before sending to Copilot API.

These keywords are internal to Anthropic's billing system and should not
be forwarded to the Copilot backend, as they may cause unexpected behavior
or errors.

Fixes #174
PR: #175

Author: jacks0n

src/routes/messages/non-stream-translation.ts

@@ -71,19 +71,42 @@ function translateAnthropicMessagesToOpenAI(
   return [...systemMessages, ...otherMessages]
 }
 
+const ANTHROPIC_RESERVED_KEYWORDS = [
+  "x-anthropic-billing-header",
+  "x-anthropic-billing",
+]
+
+function filterAnthropicReservedContent(text: string): string {
+  let filtered = text
+  for (const keyword of ANTHROPIC_RESERVED_KEYWORDS) {
+    filtered = filtered
+      .split("\n")
+      .filter((line) => !line.includes(keyword))
+      .join("\n")
+  }
+  return filtered
+}
+
 function handleSystemPrompt(
   system: string | Array<AnthropicTextBlock> | undefined,
 ): Array<Message> {
   if (!system) {
     return []
   }
 
-  if (typeof system === "string") {
-    return [{ role: "system", content: system }]
-  } else {
-    const systemText = system.map((block) => block.text).join("\n\n")
-    return [{ role: "system", content: systemText }]
+  let systemText: string
+  systemText =
+    typeof system === "string"
+      ? system
+      : system.map((block) => block.text).join("\n\n")
+
+  systemText = filterAnthropicReservedContent(systemText)
+
+  if (systemText.trim().length === 0) {
+    return []
   }
+
+  return [{ role: "system", content: systemText }]
 }
 
 function handleUserMessage(message: AnthropicUserMessage): Array<Message> {