fix: [engines] buffer request body and set Request.GetBody to support HTTP/2 GOAWAY retries

[thinker0]

Description

This pull request solves the 502 Bad Gateway error on POST/PUT requests when the upstream server returns an HTTP/2 GOAWAY frame (e.g., during keep-alive rotation or rolling updates).

Root cause:
In pkg/proxy/engines/httpproxy.go, upstream requests with bodies (like POST requests) did not define the Go standard library http.Request.GetBody field. When the Go http2.Transport (or http.Transport for reused connections) receives a GOAWAY or connection-level error, it tries to automatically retry the request, but ONLY if Request.GetBody is set. Without GetBody, the transport cannot re-read the request body, aborts the retry, and Trickster proxy returns 502 Bad Gateway (under the log signature nil response from upstream origin with detail http2: Transport: cannot retry err after Request.Body was written; define Request.GetBody to avoid this error).

Solution:
Before calling o.HTTPClient.Do(r) in PrepareFetchReader, we read the request body into a memory buffer and set r.GetBody to return a new io.ReadCloser reading from that buffer. This allows Go's HTTP client/transport to clone and re-read the body during transient network/HTTP2 connection failure retries.

Also added clean unit tests (TestPrepareFetchReader_GetBody and TestHTTP2GOAWAYRetry in httpproxy_test.go) to verify body replication and the end-to-end transport retry flow.

Type of Change

• Bug fix
• New feature
• Optimization
• Test coverage
• Documentation
• Infrastructure

AI Disclosure

• This contribution DOES include AI-generated changes, and I have reviewed the relevant contributing guidelines.