feat(body): compress request bodies

[glennawatson]

What kind of change does this PR introduce?

Feature.

What is the new behavior?

A method can send its request body under a content coding, per endpoint or per client.

• [Body(Compression = ...)] sets the coding for one method. Refit compresses whatever the serializer produced and sets Content-Encoding to match, so the coding composes with every BodySerializationMethod - JSON, JSON Lines, URL-encoded forms and raw strings alike.

  [Post("/measurements")]
  Task Upload([Body(Compression = RequestCompression.GZip)] Measurement[] batch);
  
  [Post("/measurements")]
  Task UploadSmallest([Body(
      Compression = RequestCompression.Brotli,
      CompressionLevel = CompressionLevel.SmallestSize)] Measurement[] batch);

• RefitSettings.RequestCompression sets a client-wide default. A [Body] parameter naming its own coding overrides it, and RequestCompression.None on the parameter opts one method out of a coding the settings turned on.

• The coding is applied by the framework's own wrappers where they exist. .NET 11.0 uses GZipCompressedContent, BrotliCompressedContent and ZstandardCompressedContent; older targets compress through GZipStream or BrotliStream. Both request paths - the source-generated client and the reflection request builder - go through one entry point, so they cannot diverge.

• RefitSettings.RequestCompressionOptions sets the compressor's own knobs - window size, strategy, a Zstandard dictionary - which a CompressionLevel cannot express. Options set for a coding replace the level for that coding; codings left unset still compress by level. The options types arrived with .NET 9.0 and Zstandard's with .NET 11.0, so this surface only exists from net9.0 onward.

  var settings = new RefitSettings
  {
      RequestCompression = RequestCompression.Brotli,
      RequestCompressionOptions = new()
      {
          Brotli = new() { Quality = 9, WindowLog2 = 22 },
          GZip = new() { CompressionLevel = 6 },
      },
  };

• An unavailable coding throws rather than sending the body uncompressed. PlatformNotSupportedException is raised when the request is built, naming what the target framework can do:

  Coding	Content-Encoding	Available on
  RequestCompression.GZip	gzip	every target
  RequestCompression.Brotli	br	.NET 8.0 and later
  RequestCompression.Zstandard	zstd	.NET 11.0 and later

What is the current behavior?

Refit sends every request body uncompressed.

• Compressing one required a DelegatingHandler that re-wraps content on the way out, which cannot tell one endpoint from another without inspecting the URI.

Closes #2307

What might this PR break?

None.

• The default on both [Body] and RefitSettings leaves bodies uncompressed, so an existing client sends exactly what it sent before.
• Generated clients now emit a CompressBodyContent call after the body assignment. It returns the content unchanged when no coding applies, which is what makes the settings-level default reach generated code.
• The compressed length is unknown until the body has been written, so a compressed request is sent chunked with no Content-Length.

Checklist

• I have read the Contribute guide
• Tests have been added or updated (for bug fixes / features)
• Docs have been added or updated (for bug fixes / features)
• Changes target the main branch
• PR title follows Conventional Commits

Additional information

There is no negotiation for a compressed request body, so this is opt-in and the README says to turn it on only against a server known to accept one.

The .NET 11.0 content wrappers are still preview API. RequestCompression is Refit's own enum, so the coding surface is insulated from them; RequestCompressionOptions does expose ZLibCompressionOptions, BrotliCompressionOptions and ZstandardCompressionOptions directly, which is deliberate - mirroring those knobs into Refit types would be a surface to maintain forever and would drift from the framework.

Hand-authored files worth reviewing: RequestContentCoding.cs (resolution and per-target dispatch), CompressedContent.cs (the pre-net11.0 wrapper), RequestCompressionOptions.cs, and Emitter.Inline.Content.cs (the emitted wrap). The PublicAPI.txt diffs are the analyzer's own baseline fixer output.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (74cbb64) to head (f2df89a).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #2311    +/-   ##
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  Coverage   99.91%   99.91%            
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  Files         192      194     +2     
  Lines       10027    10138   +111     
  Branches     1924     1947    +23     
========================================
+ Hits        10018    10129   +111     
  Partials        9        9            

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
98.4% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud