fix: extract Docling async markdown result

[he-yufeng]

Summary

• align the Docling async parser defaults with docling-serve v1: port 5001 examples, files upload field, task_status polling, and /v1/status/poll/{task_id}
• add a result URL template fallback for services that return only task status while exposing results at /v1/result/{task_id}
• extract document.md_content from the Docling result JSON instead of returning the whole response envelope as raw document text

Verified locally

• python -m pytest tests\test_pipeline_release_closure.py -q -k "docling or mineru_empty_service_result"
• python -m ruff check lightrag\pipeline.py tests\test_pipeline_release_closure.py
• python -m ruff format --check lightrag\pipeline.py
• python -m py_compile lightrag\pipeline.py tests\test_pipeline_release_closure.py
• git diff --check

Note: running the full tests/test_pipeline_release_closure.py file on Windows still hits two existing path-separator assertions unrelated to this change.

Closes #2996