Read every provider stream through one server-sent event reader

app/MindWork AI Studio/Provider/ServerSentEvent.cs

@@ -0,0 +1,13 @@
+namespace AIStudio.Provider;
+
+/// <summary>
+/// One event of a server-sent event stream, as it came off the wire.
+/// </summary>
+/// <remarks>
+/// The raw line travels next to its payload because not every decision can be made from the
+/// payload alone: the Responses API, for one, ends its stream with an "event:" line which carries
+/// no payload at all.
+/// </remarks>
+/// <param name="Line">The line as it arrived, including its "data:" prefix when it had one.</param>
+/// <param name="Data">The payload of a data line, empty for every other kind of line.</param>
+public readonly record struct ServerSentEvent(string Line, string Data);