[razkevich]

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Description of the pull request

The default HttpClient never sets AutomaticDecompression, so it sends no Accept-Encoding and the API returns every response uncompressed. This turns on gzip for both default handlers (HttpClientHandler and SocketsHttpHandler). A caller-supplied ClientOptions.HttpClient is untouched.

The API already serves gzip, brotli and zstd to every client that asks; only this SDK's default client doesn't ask. One server-side app using 2.1.1 pulls ~232 GB/day of QueryChannels responses (avg 334 KB) uncompressed. A live QueryChannels response measured 240 KB plain vs 17 KB gzipped.

Testing (no dotnet test changes; the existing suite needs live credentials):

• All 9 target frameworks build with 0 warnings / 0 errors.
• Local server checking the request: master sends no Accept-Encoding (14.5 KB on the wire); this branch sends gzip (388 B), decodes the body and still reads the rate-limit headers, for both the net6.0 (SocketsHttpHandler) and netstandard2.0 (HttpClientHandler) builds.
• Against the production API: QueryChannels (30 channels × 30 messages) through this branch's default client and through a plain HttpClient deserialize to identical objects, rate limits included, on both builds.

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