fix: stabilize Gemma 4 brewery generation

remove misleading turn-token output guidance from the brewery prompt
extract the last balanced JSON object before validation
keep README model setup and run instructions aligned
preserve Gemma 4 sampling defaults and local model usage

pipeline/src/data_generation/llama/helpers.cpp

@@ -263,12 +263,14 @@ static void AppendTokenPiece(const llama_vocab* vocab, llama_token token,
    output.append(buffer.data(), static_cast<std::size_t>(bytes));
 }
 
-static bool ExtractFirstJsonObject(const std::string& text,
-                                   std::string& json_out) {
+static bool ExtractLastJsonObject(const std::string& text,
+                                  std::string& json_out) {
    std::size_t start = std::string::npos;
    int depth = 0;
    bool in_string = false;
    bool escaped = false;
+   bool found = false;
+   std::string candidate;
 
    for (std::size_t i = 0; i < text.size(); ++i) {
       const char ch = text[i];
@@ -303,13 +305,27 @@ static bool ExtractFirstJsonObject(const std::string& text,
          }
          --depth;
          if (depth == 0 && start != std::string::npos) {
-            json_out = text.substr(start, i - start + 1);
-            return true;
+            candidate = text.substr(start, i - start + 1);
+            found = true;
          }
       }
    }
 
-   return false;
+   if (!found) {
+      return false;
+   }
+
+   json_out = std::move(candidate);
+   return true;
+}
+
+std::string ExtractLastJsonObjectPublic(const std::string& text) {
+   std::string extracted;
+   if (ExtractLastJsonObject(text, extracted)) {
+      return extracted;
+   }
+
+   return {};
 }
 
 static std::string ValidateBreweryJson(const std::string& raw,
@@ -371,7 +387,7 @@ static std::string ValidateBreweryJson(const std::string& raw,
    std::string validation_error;
    if (ec) {
       std::string extracted;
-      if (!ExtractFirstJsonObject(raw, extracted)) {
+      if (!ExtractLastJsonObject(raw, extracted)) {
          return "JSON parse error: " + ec.message();
       }