Support parsing DAG JSON from <json>...</json> tags and json code blocks to improve the robustness of the parse_dag function.

[iejzh]

During testing with the Qwen2.5-VL model, the JSON output is mostly wrapped inside json code blocks rather than the expected ... tag format. Currently, the parse_dag function only extracts JSON from ... tags, causing it to fail to parse the model output correctly.

Issue Description:

• Qwen2.5-VL model often outputs JSON wrapped in json code blocks
• The function only recognizes JSON inside ... tags, resulting in parsing failures

Please optimize the parse_dag function to support the Qwen2.5-VL model’s output formats, improving overall stability and user experience.