feat: support Server-Sent Events as a streaming content format

[glennawatson]

What kind of change does this PR introduce?

Feature - adds text/event-stream (Server-Sent Events) as a first-class streaming content format.

What is the new behavior?

A Refit method declared as IAsyncEnumerable<T> against an SSE endpoint now yields one T per SSE data event, streamed live and unbuffered.

• New StreamingContentFormat.ServerSentEvents enum member.
• DetectStreamingFormat maps a text/event-stream response content type to the new format.
• SystemTextJsonContentSerializer parses each SSE event with SseParser and deserializes the event's data payload to T (the event type/id envelope is dropped), reusing the same source-generated / reflection JsonTypeInfo<T> path the serializer already uses, so it stays trim and Native AOT clean.
• Both the generated and reflection execution paths inherit this through the shared streaming seam -> no generator or reflection-path change.
• References the System.Net.ServerSentEvents package on the targets that lack it in-box (net4x / netstandard2.0 / net8.0 / net9.0); net10.0+ use the framework reference. This gives uniform behavior on every target framework.

public interface IChatApi
{
    [Get("/chat/stream")]
    IAsyncEnumerable<ChatToken> StreamAsync();
}

await foreach (var token in api.StreamAsync())
{
    // one ChatToken per `data:` event, streamed live
}

What is the current behavior?

Consuming SSE already worked but only by returning Task<Stream> and feeding it to System.Net.ServerSentEvents.SseParser manually. There was no typed, per-event streaming experience for SSE endpoints.

Closes #2228

What might this PR break?

None expected. The change is additive: a new StreamingContentFormat.ServerSentEvents enum member and a System.Net.ServerSentEvents package reference on the targets that lack it in-box. Existing content formats and code are unchanged. If you need the raw event envelope (event type or id), return Task<Stream> and use System.Net.ServerSentEvents.SseParser directly.

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

• Follow-up to feature: Support for SSE #1291; closes the ergonomic seam tracked in feature: first-class IAsyncEnumerable<T> consumption of Server-Sent Events (text/event-stream) #2228. SSE is simply one more frame format alongside JSON arrays and JSON Lines, slotting into the existing streaming seam with no new subsystem.
• Tests: SSE streaming through the generated path and the reflection (dynamic-route) path; a parity test proving both paths yield the same sequence; coverage of the default (reflection) serializer and the source-generated context serializer; the DetectStreamingFormat text/event-stream branch and the serializer SSE branch.
• Verification: dotnet build Refit.slnx -c Release (all target frameworks) -> 0 warnings, 0 errors; full Refit.Tests (net8.0) passes; generator tests pass with no snapshot change; 100% patch coverage on the changed lines.
• Docs: README streaming section documents SSE consumption; docs/breaking-changes.md notes the new enum member and package reference.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (759a3c0) to head (730f8b6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #2232   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          182       182           
  Lines         9444      9453    +9     
  Branches      1772      1774    +2     
=========================================
+ Hits          9444      9453    +9     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud