🐛 reject failed stopword downloads

[ump45nose]

Summary

• reject non-success HTTP responses before writing the stopword file
• avoid treating a short 404/500 response body as a valid stopword list
• add a focused regression test that verifies an HTTP error returns False and leaves no file

Validation

• direct fail-before/pass-after mocked HTTP reproduction
• pytest -q test/sdk/core/nlp/test_stopwords.py — 1 passed
• Ruff check and format check on both changed files
• git diff --cached --check