feat(sdk): add streaming download_to_file for devbox files

[jrvb-rl]

Summary

devbox.file.download() reads the entire file into memory (response.read()), the mirror image of the upload-side buffering. This adds download_to_file(dest) on both the sync and async FileInterface that streams the response body straight to disk in chunks via with_streaming_response, so large downloads never have to fit in RAM.

The existing download() (returns bytes) is kept unchanged for small-file convenience — streaming into a bytes return is impossible by definition, so this is additive rather than a behavior change.

Companion to #815 (streaming uploads).

Changes

• sdk/devbox.py / sdk/async_devbox.py: new download_to_file(dest, *, chunk_size=None, **params) using with_streaming_response.download_file(...) + stream_to_file.
• Tests: test_download_to_file (sync + async) assert the body is streamed to disk and the correct path is forwarded.

Usage

devbox.file.download_to_file("local_output.bin", path="/home/user/output.bin")
# async:
await devbox.file.download_to_file("local_output.bin", path="/home/user/output.bin")

Test plan

• uv run pytest tests/sdk/devbox tests/sdk/async_devbox — 95 passed
• ruff check clean

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