fix(client): handle multipart and streaming bodies on paid 402 retry

[bennytimz]

Summary

This PR complements #156 (bytes/content= body replay) and addresses the two body-type gaps that #156 does not cover.

What #156 fixed

PR #156 by @Tleaoo fixes the core bug: PaymentTransport was reusing the already-consumed request.stream on 402 retry, so any POST/PUT/PATCH body was silently dropped. The fix is await request.aread() before dispatch and content=request.content (instead of stream=request.stream) on retry.

What this PR adds on top

1. Multipart / files= bodies (tested)

aread() works for MultipartStream because it implements both SyncByteStream and AsyncByteStream. The full multipart-encoded bytes are buffered before the first dispatch, and the paid retry sends the identical payload.

New tests:

• test_paid_retry_replays_multipart_body — files= upload, verifies body and filename are present in the retry
• test_paid_retry_replays_put_body — PUT with a bytes body
• test_paid_retry_replays_patch_body — PATCH with a bytes body

Also includes test_paid_retry_replays_request_body (same as #156's test) so this branch is self-contained.

2. Async generator bodies — explicit PaymentError

content=async_gen() in httpx produces an AsyncIteratorByteStream which is AsyncByteStream but not SyncByteStream. Unlike MultipartStream and ByteStream, an async generator:

• may be infinite or tied to a one-shot I/O source
• may already be partially consumed before the transport receives it
• silently buffering it would break the streaming contract and could OOM

PaymentTransport.handle_async_request now detects this case before any I/O and raises PaymentError with a clear message directing callers to use a buffered body instead.

New test:

• test_paid_retry_raises_for_streaming_body — verifies PaymentError is raised immediately with no requests sent

Detection logic

if isinstance(request.stream, httpx.AsyncByteStream) and not isinstance(
    request.stream, httpx.SyncByteStream
):
    raise PaymentError("Streaming request bodies (async generators) are not supported ...")

ByteStream and MultipartStream are both Sync+Async so they pass through. Only AsyncIteratorByteStream (async generators) is async-only and is caught.

Test plan

• All 28 tests/test_client.py tests pass (confirmed locally)
• No regressions in the rest of the test suite
• Multipart body is identical byte-for-byte on the initial and retry requests
• Async generator body raises PaymentError before any network I/O