`RecordingClient` breaks requests with non-streaming request bodies (HTTP 400 from ELB)

[signadou]

Summary

aws_smithy_http_client::test_util::dvr::RecordingClient can cause real AWS SDK requests with non-streaming request bodies to fail (observed as 400 Bad Request from awselb/2.0). The same request succeeds when using the standard client without RecordingClient.

Reproduction

• Use RecordingClient as the SDK HTTP client (via aws_config::defaults(...).http_client(recording_client.clone())).
• Call an operation with a small JSON request body (non-streaming; SdkBody::bytes() is available), e.g. Bedrock Runtime converse_stream.

Observed failure:

• Response is 400 Bad Request with content-type: text/html from awselb/2.0
• The request is correctly signed and has Content-Length, etc.
• If request body recording is disabled (i.e., don't swap the request body), the request succeeds.

Root cause

RecordingClient records request bodies by replacing the body with a channel-backed streaming body and spawning a task to forward data from the original body into that channel. For non-streaming request bodies, this channel-based approach can cause timing/framing issues (particularly with HTTP/2), leading to malformed requests on the wire.

Proposed fix

For non-streaming request bodies (SdkBody::bytes() returns Some), buffer the bytes up front:

1. Copy the bytes.
2. Record them into the DVR event stream.
3. Replace the request body with a new SdkBody created from the same bytes.

For streaming request bodies, keep the existing channel-based path.

Additionally, preserve size hints for the channel-backed body by passing through content_length() to keep the replacement body's size_hint() consistent with Content-Length.

Notes

• Response body recording continues to use the channel-based approach (works fine since we're consuming, not producing).
• This is in test-util DVR code; no change to production client stacks.

---

I've already fixed the issue locally and I'll be making a PR momentarily.