Enable CURL response decompression

## Problem

The CURL client currently passes `NULL` to `CURLOPT_ACCEPT_ENCODING`, which means it does not advertise any supported compression encodings in the `Accept-Encoding` request header. As a result, servers never send compressed responses, leading to unnecessarily high bandwidth usage.

## Proposed Solution

Change `CURLOPT_ACCEPT_ENCODING` from `NULL` to `""` (empty string). Per the [CURL documentation](https://curl.se/libcurl/c/CURLOPT_ACCEPT_ENCODING.html), passing an empty string causes CURL to:

1. Advertise all compression encodings it supports (e.g. gzip, deflate, br, zstd) via the `Accept-Encoding` header
2. Automatically decompress responses that use those encodings

This is a one-line change that can significantly reduce bandwidth for HTTP transfers.

Locate the CURL client setup where CURLOPT_ACCEPT_ENCODING is currently passed NULL. Change that setting to an empty string, then verify that the client advertises supported compression encodings and automatically decompresses compressed responses; the issue does not name a specific file or test.

Written by the indexing model from the issue text.