1. update .python-version from 3.11 to 3.13
2. add tests for IS_RUNNING and DIR_EXISTS conditions
3. add graph-related tests including string ref parsing, mermaid output, GraphComposer and compose function
4. add storage backend TTL tests for both MemoryBackend and JSONBackend
5. add new system task tests for clr, reset_icon_cache, setenv and which
6. add comprehensive task spec tests including soft dependencies, retry policy, context managers and task template
7. add executor edge case tests for various scenarios
zhou