Fix RecordingClient request body recording for non‑streaming bodies

[signadou]

Motivation and Context

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.

Closes #4495

Description

For non‑streaming request bodies (where SdkBody::bytes() is available), we now buffer the bytes, record them, and rebuild the request body from the buffered data instead of swapping in a channel‑based streaming body. This avoids timing issues that can break HTTP/2 framing and cause 400 responses from ELB.

Also preserves size hints on channel bodies by carrying over the original content length, keeping body metadata consistent with Content-Length.

Note: This change is limited to the DVR test utility (aws-smithy-http-client test_util::dvr) and does not affect default/prod HTTP clients.

Testing

• Ran cargo test -p aws-smithy-http-client --features "test-util,legacy-rustls-ring" dvr inside rust-runtime and confirmed the tests still pass.
• Updated real project to use patched crate and confirmed the problem was fixed.

Checklist

• For changes to the smithy-rs codegen or runtime crates, I have created a changelog entry Markdown file in the .changelog directory, specifying "client," "server," or both in the applies_to key.

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[landonxjames]

This generally looks good to me. There is one CI failure related to rustfmt issues, tried to fix it myself, but I don't seem to have permissions to push to your branch. Can approve once that is fixed. Note you'll also need to bump the patch number in aws-smithy-http-client/Cargo.toml since that is going to fail a later check we run.

[signadou]

@landonxjames Thank you for reviewing!  I think you're not able to push because I forked this repo into the Piebald-AI organization instead of into my personal account.  I'll make sure to use a personal fork for PRs in the future. I've fixed the formatting and bumped the patch version.