Codex CLI 0.125+ defaults to sending request bodies with
Content-Encoding: zstd. Without server-side decompression the gateway
returns 'Failed to parse request body' on /v1/responses (and any other
JSON endpoint) because gjson sees raw zstd bytes.
ReadRequestBodyWithPrealloc now inspects Content-Encoding and
transparently decodes zstd, gzip/x-gzip, and deflate bodies before
returning them, then strips the encoding headers and updates
ContentLength so downstream code can reuse the bytes safely.
Unsupported encodings produce a clear error.
Adds unit tests covering identity, zstd, gzip, deflate, unsupported
encoding, corrupt zstd payloads, nil bodies, and explicit identity.
