Fix hangs with large request payloads

[clanky1024]

Summary

• drain request bodies before writing the configured response
• add regression coverage for small and 20 MiB payloads

Problem

The request handler did not read the request body. For large HTTP/1 requests, this could cause the server to close the connection with unread data or wait while net/http attempted to drain a chunked body, resulting in stalled or hanging clients.

Solution

Stream the request body to io.Discard before writing the response. This consumes large payloads without buffering them in memory and allows the request to complete normally.

Testing

• go test ./...
• confirmed the regression test fails against the original implementation
• verified complete known-length and chunked 20 MiB uploads

[hashicorp-cla-app]

All committers have signed the CLA.

[hashicorp-cla-app]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

Learn more about why HashiCorp requires a CLA and what the CLA includes

_{Have you signed the CLA already but the status is still pending? Recheck it.}