[michboy]

Summary

Fixes #823.

The LFM2 tool-call parser can enter an infinite loop when processing malformed arguments with mismatched closing brackets, potentially causing unbounded memory growth.

This PR prevents the parser from getting stuck on malformed input.

Changes

• Handle mismatched closing brackets in py_literal_to_json().
• Return null for empty scalar values.
• Skip unexpected closing brackets in extract_lfm2_tool_calls().
• Ensure the parser makes forward progress on malformed arguments.
• Add four regression test cases for valid and malformed tool calls.

Testing

Added cactus-engine/tests/test_chat_tools_823.cpp with four standalone C++ regression tests.

Local test results pending verification.

Attribution

The fix follows the approach originally proposed by @Rasaboun in #823, with regression tests added.