Add DecompressionMethods.Zstandard for HTTP automatic decompression

[Copilot]

Description

Adds Zstandard compression support to DecompressionMethods for automatic HTTP response decompression, matching the API proposal. Follows the established pattern from Brotli support.

Changes

API Surface

• Added Zstandard = 0x8 to DecompressionMethods enum in System.Net.Primitives

Implementation

• DecompressionHandler: Added "zstd" encoding support with ZstandardEnabled property and ZstandardDecompressedContent class using ZstandardStream
• DecompressionHandler: Optimized Accept-Encoding header construction to use Span<string?> instead of List<string?> for better performance and reduced heap allocations
• SocketsHttpHandler: Updated SupportedDecompressionMethods to include Zstandard
• Added project reference to System.IO.Compression.Zstandard

Tests

• Added Zstandard test cases to HttpClientHandlerTest.Decompression.cs covering decompression, empty bodies, Accept-Encoding headers, and quality weightings

Usage

var handler = new HttpClientHandler
{
    AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Zstandard
};

using var client = new HttpClient(handler);
// Automatically decompresses zstd-encoded responses
var response = await client.GetAsync("https://example.com");

Notes

• Zstandard support requires System.IO.Compression.Zstandard which is already available
• Not supported on WinHttpHandler (consistent with Brotli)
• Accept-Encoding header includes "zstd" when enabled
• Accept-Encoding header construction optimized using Span<string?> to avoid heap allocations

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[API Proposal]: DecompressionMethods.Zstd for web requests</issue_title>
<issue_description>### Background and motivation

DecompressionMethods auto-decoding was last expanded with Brotli. Since then, some web servers started serving content using ZSTD compression instead. As such, it'd be appropriate for .NET to include the ability to handle such encoding out of the box.

CURL supports this starting with version 7.72.0, released in August 2020.
Cloudflare supports this since last September.

API Proposal

namespace System.Net;

[Flags]
public enum DecompressionMethods
{
    None = 0,
    All = ~None
    GZip = 0x1,
    Deflate = 0x2,
    Brotli = 0x4,
+   Zstandard = 0x8,
}

API Usage

var handler = new HttpClientHandler
{
    AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Zstandard
};

Alternative Designs

Zstd instead of Zstandard?

The name used for encoding headers is zstd and the media type is application/zstd, while the proposed compression APIs are using Zstandard.

Similar with Brotli where br is used for headers, but we call the APIs Brotli.

Risks

Requires shipping Zstd in System.Compression.Native.</issue_description>

Comments on the Issue (you are @copilot in this section)

@stephentoub This is blocked on https://github.com//issues/59591. @MihaZupan Seems like a reasonable thing to add whenever #59591 happens @bartonjs [Video](https://www.youtube.com/watch?v=NzqIFvMFxWg&t=1h4m36s)

• We think Zstandard is a better name that Zstd

namespace System.Net;

[Flags]
public enum DecompressionMethods
{
    None = 0,
    All = ~None
    GZip = 0x1,
    Deflate = 0x2,
    Brotli = 0x4,
+   Zstandard = 0x8,
}
```</body></comment_new>
</comments>

• Fixes [API Proposal]: DecompressionMethods.Zstd for web requests #112075

---

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[rzikm]

@copilot Please review my comments

[Copilot]

@copilot Please review my comments

I've reviewed your comment about adding the project reference to System.IO.Compression.Zstandard. This was addressed in commit e2b4db7.

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[Copilot]

Pull request overview

Adds Zstandard (zstd) support to HttpClientHandler/SocketsHttpHandler automatic response decompression by extending DecompressionMethods and wiring the new encoding into the existing decompression pipeline (mirroring the Brotli pattern).

Changes:

• Extended System.Net.DecompressionMethods with Zstandard = 0x8 (and updated reference assembly).
• Added zstd handling in DecompressionHandler and enabled it in SocketsHttpHandler’s supported decompression mask.
• Added System.IO.Compression.Zstandard dependency and expanded decompression/Accept-Encoding functional tests to cover Zstandard.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/libraries/System.Net.Primitives/src/System/Net/DecompressionMethods.cs	Adds DecompressionMethods.Zstandard enum value.
src/libraries/System.Net.Primitives/ref/System.Net.Primitives.cs	Updates public ref API to include Zstandard.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/SocketsHttpHandler.cs	Includes Zstandard in SupportedDecompressionMethods.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/DecompressionHandler.cs	Adds zstd Accept-Encoding emission and ZstandardStream-based decompression.
src/libraries/System.Net.Http/src/System.Net.Http.csproj	Adds project reference to System.IO.Compression.Zstandard.
src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.Decompression.cs	Adds Zstandard test coverage for decompression and Accept-Encoding behaviors.

[rzikm]

@jkotas raised a good point in a related issue about trimming. #123532 (comment) I believe the same argument would apply here.

[stephentoub]

@jkotas raised a good point in a related issue about trimming. #123532 (comment) I believe the same argument would apply here.

Yes and no.

If you use AutomaticDecompression, then regardless of which compression algorithms you specify, you'll root them all.

But if you don't set AutomaticDecompression, the whole compression support becomes trimmable. @eerhardt did that a few years ago in #78198.

[Copilot]

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[rzikm]

/ba-g Test failures are unrelated