fix(backend): increase max_output_tokens to 65536, log finish_reason on error

Replace truncation-recovery heuristic with a higher token budget.
On JSON parse failure, log finish_reason and 160-char error context
to make the root cause visible.

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

backend/innercontext/api/products.py

@@ -360,13 +360,18 @@ 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=8192,
+            max_output_tokens=65536,
             temperature=0.0,
         ),
     )
+    candidate = response.candidates[0] if response.candidates else None
+    finish_reason = str(candidate.finish_reason) if candidate else "unknown"
     raw = response.text
     if not raw:
-        raise HTTPException(status_code=502, detail="LLM returned an empty response")
+        raise HTTPException(
+            status_code=502,
+            detail=f"LLM returned an empty response (finish_reason={finish_reason})",
+        )
     # Fallback: extract JSON object in case the model adds preamble or markdown fences
     if not raw.lstrip().startswith("{"):
         start = raw.find("{")
@@ -380,7 +385,12 @@ def parse_product_text(data: ProductParseRequest) -> ProductParseResponse:
     try:
         parsed = json.loads(raw)
     except json.JSONDecodeError as e:
-        log.error("Gemini parse-text raw response (failed):\n%s", raw)
+        log.error(
+            "Gemini parse-text JSON error at pos %d finish_reason=%s context=%r",
+            e.pos,
+            finish_reason,
+            raw[max(0, e.pos - 80) : e.pos + 80],
+        )
         raise HTTPException(status_code=502, detail=f"LLM returned invalid JSON: {e}")
     try:
         return ProductParseResponse.model_validate(parsed)