Adding SSE support

[carldebilly]

This PR adds comprehensive Server-Sent Events (SSE) support to the Yllibed.HttpServer library. The implementation includes SSE session management, helper utilities, content negotiation, and streaming response infrastructure. The PR also modernizes the project structure by introducing central package management, updating test frameworks, and adding new solution file format support.

• Adds complete SSE implementation with SseSession, SseHandler, and helper classes
• Introduces streaming response infrastructure to support SSE and other streaming scenarios
• Modernizes build system with central package management and updated dependencies

[Copilot]

Pull Request Overview

This PR adds comprehensive Server-Sent Events (SSE) support to the Yllibed.HttpServer library. The implementation includes SSE session management, helper utilities, content negotiation, and streaming response infrastructure. The PR also modernizes the project structure by introducing central package management, updating test frameworks, and adding new solution file format support.

• Adds complete SSE implementation with SseSession, SseHandler, and helper classes
• Introduces streaming response infrastructure to support SSE and other streaming scenarios
• Modernizes build system with central package management and updated dependencies

Reviewed Changes

Copilot reviewed 49 out of 51 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Yllibed.HttpServer/Sse/*.cs	New SSE implementation classes for sessions, handlers, and helpers
Yllibed.HttpServer/Server.HttpServerRequest.cs	Added streaming response support and modernized async patterns
Yllibed.HttpServer/IHttpServerRequest.cs	Added SetStreamingResponse method signature
Yllibed.HttpServer/Extensions/*.cs	New Accept header validation and modernized extension methods
Yllibed.HttpServer/Helpers/AcceptHeaderHelper.cs	New Accept header parsing according to HTTP specs
Test files	Comprehensive SSE test coverage and modernized test infrastructure
Project files	Central package management, updated dependencies, and solution modernization

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[Copilot]

This loop only uses the first value and then breaks immediately. Consider using FirstOrDefault() for clarity: _lastEventId = values.FirstOrDefault()?.Trim();

[Copilot]

The FlushAsync method ignores the cancellation token parameter. In .NET Standard 2.0, TextWriter.FlushAsync() doesn't accept a CancellationToken, but the extension method should acknowledge this limitation or throw if the token is already cancelled.

Suggested change

	public static Task FlushAsync(this TextWriter writer, CancellationToken ct = default) => writer.FlushAsync();
	public static Task FlushAsync(this TextWriter writer, CancellationToken ct = default)
	{
	if (ct.IsCancellationRequested)
	return Task.FromCanceled(ct);
	return writer.FlushAsync();
	}

[Copilot]

Similar to FlushAsync, this ReadLineAsync method ignores the cancellation token parameter. The .NET Standard 2.0 version doesn't support CancellationToken, but the method signature suggests it should be honored.

Suggested change

	public static Task<string> ReadLineAsync(this TextReader reader, CancellationToken ct = default) => reader.ReadLineAsync();
	public static Task<string> ReadLineAsync(this TextReader reader, CancellationToken ct = default)
	{
	if (ct.IsCancellationRequested)
	return Task.FromCanceled<string>(ct);
	return reader.ReadLineAsync();
	}