Add line and Server-Sent Events readers for streaming responses

[Mako88]

Summary

Adds two streaming-consumption methods on ISimpleStreamResponse, so consumers don't have to hand-roll SSE framing on top of the raw Body stream:

• ReadLinesAsync(ct) — reads the body as text lines (CR/LF/CRLF), encoding from the Content-Type charset (UTF-8 default), cancellation observed mid-read.
• ReadServerSentEventsAsync(ct) — parses the text/event-stream wire format per the WHATWG SSE spec: blank-line dispatch, multiple data: lines joined with newlines, comment/keep-alive (:) lines skipped, event/id/retry fields, and id carrying forward. Returns ServerSentEvent frames (Data, EventType, Id, Retry).

Design

• Interface methods, not extension methods — declared on ISimpleStreamResponse, implemented in SimpleStreamResponse, so callers can mock them (a test asserts this).
• SSE framing is a web standard (the same format browser EventSource consumes), so it belongs in the library. Application-specific conventions are deliberately left to the caller:
  • No [DONE] special-casing — it passes through as ordinary Data (OpenAI's convention; Anthropic uses event: message_stop). The caller's if decides.
  • No payload deserialization — Data is a string; the caller deserializes with whatever serializer/type it wants.

using var response = await client.MakeStreamRequest(request, ct);
await foreach (var sse in response.ReadServerSentEventsAsync(ct))
{
    if (sse.Data == "[DONE]") break;            // caller's convention
    var chunk = client.Serializer.Deserialize<MyChunk>(sse.Data);
}

Notes

• IAsyncEnumerable works on netstandard2.0 via Microsoft.Bcl.AsyncInterfaces, already pulled in transitively by System.Text.Json — no new dependency. Added <LangVersion>latest</LangVersion> so the netstandard2.0 target (C# 7.3 by default) can compile async streams.
• Composes with the existing CancellationAwareStream, so cancellation is honored mid-read.
• Backward-compatible, additive only → minor bump.

Tests

14 new tests (parser units via in-memory streams, a WireMock end-to-end through MakeStreamRequest, and a mockability test): line splitting, CRLF, multi-data join, no-space-after-colon, comment skipping, event/id/retry, id-carries-forward, reset-between-events, [DONE] pass-through, event-without-data not dispatched, incomplete trailing event not dispatched, pre-cancelled token throws, mockable interface. net10.0 and net48: 110 each.

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