The prior implementation routed download_to_file through the shared
_request() path, which uses httpx.AsyncClient.request() inside a
context manager that closes before aiter_bytes() iterates. The body
was read into memory first and the chunked write loop replayed it
from buffer. On small test payloads this was invisible; on real
Teams meeting recordings (hundreds of MB) it would force the full
artifact into RAM per download.
Rewrites download_to_file to open its own AsyncClient and use
client.stream(), keeping the context open across the aiter_bytes
iteration so the body is actually streamed chunk-by-chunk to disk.
Retry/token-refresh/Retry-After semantics are preserved by handling
them inline on the stream path. Partial .part files are cleaned up
on transport errors and on exhausted retries.
Adds three tests: large-payload streaming verifies the chunk loop
runs multiple times (discriminator: 512 KiB at chunk_size=65536
yields 8 chunks under streaming, 1 under buffering), transient-5xx
retry recovers after a single retry, and exhausted-retry cleans up
the partial file.
