fix: fixes Request body being released too early

[dom96]

It looks like we need to use create_proxy here to ensure the body isn't freed too early.

Test Plan

cd packages/runtime-sdk
uv run pytest 'tests/test_fastapi.py::TestPLATFORM_IO::test_upload_spills_binary_file_to_virtual_disk[3.13]' -v
uv run pytest 'tests/test_in_workerd.py::test_in_workerd[sdk-3.13]' -v

[ask-bonk]

ProviderModelNotFoundError

github run

[ask-bonk]

@dom96 Bonk workflow failed. Check the logs for details.

View workflow run · To retry, trigger Bonk again.

[ryanking13]

Thanks!