fix(backend): handle invalid/empty JSON from Gemini in product parse endpoint

- Increase max_output_tokens 4096 → 8192 to prevent truncated JSON on
  products with long INCI lists
- Return explicit 502 when response.text is None (safety filter blocks)
- Fallback JSON extraction strips markdown fences or leading preamble

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/innercontext/api/products.py

@@ -356,12 +356,21 @@ def parse_product_text(data: ProductParseRequest) -> ProductParseResponse:
         config=genai_types.GenerateContentConfig(
             system_instruction=_product_parse_system_prompt(),
             response_mime_type="application/json",
-            max_output_tokens=4096,
+            max_output_tokens=8192,
             temperature=0.0,
         ),
     )
+    raw = response.text
+    if not raw:
+        raise HTTPException(status_code=502, detail="LLM returned an empty response")
+    # Fallback: extract JSON object in case the model adds preamble or markdown fences
+    if not raw.lstrip().startswith("{"):
+        start = raw.find("{")
+        end = raw.rfind("}")
+        if start != -1 and end != -1:
+            raw = raw[start : end + 1]
     try:
-        parsed = json.loads(response.text)
+        parsed = json.loads(raw)
     except (json.JSONDecodeError, Exception) as e:
         raise HTTPException(status_code=502, detail=f"LLM returned invalid JSON: {e}")
     try: