Fix HTTPResponse cleanup for owl imports

[gaoflow]

Summary

• stop passing the raw HTTPResponse.fp object into RDF parsing for HTTP RDF sources
• let the owning HTTPResponse close itself after buffering non-seekable responses
• add a local HTTP owl:imports regression test for issue owl:imports gives ValueError: I/O operation on closed file #319

Tests

• uv run --python 3.14 --with-editable . --with pytest pytest -q test/issues/test_319.py
• uv run --python 3.11 --with-editable . --with pytest pytest -q test/issues/test_319.py
• uv run --python 3.14 --with 'ruff<0.10,>=0.9.3' ruff check pyshacl/rdfutil/load.py test/issues/test_319.py
• uv run --python 3.14 --with 'ruff<0.10,>=0.9.3' ruff format --check --no-preview --target-version py39 pyshacl/rdfutil/load.py test/issues/test_319.py
• uv run --python 3.11 --with-editable . --with pytest pytest -q test/issues/test_098.py test/issues/test_281/test_281.py test/issues/test_319.py
• uv run --python 3.11 --with-editable . --with pytest pytest -q test/test_extra.py::test_owl_imports

Closes #319.

AI assistance was used under my direction.