[API Proposal]: Add MediaTypeNames.Text.EventStream for server-sent events

[stephentoub]

Background and motivation

SSE has become more popular, especially as it's been used for streaming with AI services. #98105 is adding a parser for SSE. If you get an HTTP response with a "text/event-stream" MIME type, that indicates the response body should be treated as SSE.

API Proposal

namespace System.Net.Mime;

public static class MediaTypeNames
{
    public static class Text
    {
+       public const string EventStream = "text/event-stream";
        ...
    }
}

API Usage

n/a

Alternative Designs

n/a

Risks

n/a

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[ManickaP]

We could probably merge this into #95446 where we're collecting new additions.

[MihaZupan]

Superseded by #95446