- Add TestAIAgentConcurrentExecution with 8 integration tests exercising
_execute_tool_calls_concurrent through AIAgent for 2/3/4-tool batches,
pass-rate reporting, and Gemma 4-style read patterns.
- Fix test_malformed_json_args_forces_sequential: use JSON array '[1,2,3]'
instead of unrepairable garbage now that repair_and_load_json handles
most malformed input.
- Fix test_concurrent_handles_tool_error: replace racy call_count list
with deterministic failure based on tool_call_id to eliminate flaky
failures under ThreadPoolExecutor.
Closes#798
