fix(backend): drop response_mime_type=application/json to avoid constrained decoding

Constrained decoding is ~10x slower and consumes hidden tokens for constraint
processing, causing truncation at ~1000 chars even with 8192 max_output_tokens.
The system prompt already instructs the model to output raw minified JSON; our
NaN/markdown-fence sanitisation handles edge cases.

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

backend/innercontext/api/products.py

@@ -359,8 +359,7 @@ def parse_product_text(data: ProductParseRequest) -> ProductParseResponse:
         contents=f"Extract product data from this text:\n\n{data.text}",
         config=genai_types.GenerateContentConfig(
             system_instruction=_product_parse_system_prompt(),
-            response_mime_type="application/json",
-            max_output_tokens=65536,
+            max_output_tokens=8192,
             temperature=0.0,
         ),
     )