fix(engine): find a message's end without counting braces inside strings

cactus-engine/src/utils.h

@@ -1003,13 +1003,7 @@ inline std::vector<cactus::engine::ChatMessage> parse_messages_json(const std::s
         cactus::engine::ChatMessage msg;
 
         size_t obj_start = pos;
-        int brace_count = 1;
-        size_t obj_end = obj_start + 1;
-        while (obj_end < json.length() && brace_count > 0) {
-            if (json[obj_end] == '{') brace_count++;
-            else if (json[obj_end] == '}') brace_count--;
-            obj_end++;
-        }
+        size_t obj_end = find_matching_delimiter(json, obj_start, '{', '}');
 
         size_t role_pos = json.find("\"role\"", pos);
         if (role_pos == std::string::npos || role_pos >= obj_end) break;

cactus-engine/tests/test_messages_json_braces.cpp

@@ -0,0 +1,33 @@
+#include "test_utils.h"
+#include "../src/utils.h"
+
+using cactus::ffi::parse_messages_json;
+
+bool test_unbalanced_brace_in_content_keeps_later_messages() {
+    std::vector<std::string> images;
+    auto messages = parse_messages_json(
+        R"([{"role":"user","content":"why does `if (x) {` not compile?"},)"
+        R"({"role":"assistant","content":"it is missing its closing brace"},)"
+        R"({"role":"user","content":"thanks"}])", images);
+    return messages.size() == 3
+        && messages[1].role == "assistant" && messages[1].content == "it is missing its closing brace"
+        && messages[2].role == "user" && messages[2].content == "thanks";
+}
+
+bool test_images_stay_with_their_message() {
+    std::vector<std::string> images;
+    auto messages = parse_messages_json(
+        R"([{"role":"user","content":"what is {"},)"
+        R"({"role":"user","content":"describe this","images":["a.png"]}])", images);
+    return messages.size() == 2
+        && messages[0].images.empty()
+        && messages[1].images.size() == 1 && images.size() == 1;
+}
+
+int main() {
+    TestUtils::TestRunner runner("Message Boundary Tests");
+    runner.run_test("unbalanced_brace_in_content_keeps_later_messages", test_unbalanced_brace_in_content_keeps_later_messages());
+    runner.run_test("images_stay_with_their_message", test_images_stay_with_their_message());
+    runner.print_summary();
+    return runner.all_passed() ? 0 : 1;
+}