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

[breken-ai]

What

parse_messages_json found the end of each message object by counting every { and } in the JSON, including those inside strings. This PR uses find_matching_delimiter instead, which skips string contents and already bounds tool_calls in the same function. It changes one line.

Why

When a message's text holds an unmatched brace, its object never closes and runs to the end of the array. In coding chats that is common, for example "why does if (x) { not compile?". Every later message is dropped, and their images/audio are attached to the earlier message. Parsed on main (fa094ce8):

[{"role":"user","content":"why does `if (x) {` not compile?"},
 {"role":"assistant","content":"it is missing its closing brace"},
 {"role":"user","content":"thanks"}]
-> 1 message (the assistant turn and the latest user turn are gone)

[{"role":"user","content":"what is {"},
 {"role":"user","content":"describe this","images":["a.png"]}]
-> 1 message, and it carries a.png

The model then answers an earlier turn and never sees the latest question.

Tests

I added cactus-engine/tests/test_messages_json_braces.cpp. It needs no model.

• On main, both tests fail: unbalanced_brace_in_content_keeps_later_messages and images_stay_with_their_message.
• With this PR, both pass.
• All engine test targets build on macOS arm64. test_bucket (10/10) and test_kv_compress (29/29) still pass. The model-backed suites were not run: they need a bundle.

This touches a different hunk of parse_messages_json than #764, so the two apply independently.

AI disclosure

I found and fixed this with help from an AI coding assistant. I checked the code path by hand and reproduced the failure on main before writing the fix. The commit is signed off for the DCO.

🤖 Generated with Claude Code