feat: AI SDK compatibility - Accept header & JSON markdown stripping

- Respect Accept: application/json header to return non-streaming JSON
  instead of SSE, fixing AI SDK generateObject/generateText compatibility
- Strip markdown code block markers (```json...```) from Claude
  non-streaming responses to prevent JSON parse errors

Cherry-picked and adapted from PR #290 by @rothnic
https://github.com/decolua/9router/pull/290

Made-with: Cursor

open-sse/handlers/chatCore/nonStreamingHandler.js

@@ -77,8 +77,11 @@ export function translateNonStreamingResponse(responseBody, targetFormat, source
     const toolCalls = [];
 
     for (const block of responseBody.content) {
-      if (block.type === "text") textContent += block.text;
-      else if (block.type === "thinking") thinkingContent += block.thinking || "";
+      if (block.type === "text") {
+        // Strip markdown code block markers (e.g. kimi wraps JSON in ```json...```)
+        const text = block.text.replace(/^\s*```\s*json\s*\n?/i, "").replace(/\n?\s*```\s*$/i, "");
+        textContent += text;
+      } else if (block.type === "thinking") thinkingContent += block.thinking || "";
       else if (block.type === "tool_use") {
         toolCalls.push({ id: block.id, type: "function", function: { name: block.name, arguments: JSON.stringify(block.input || {}) } });
       }