fix: avoid setting User-Agent header in .NET Framework

[deleteLater]

fixed #24

caused by 3dd26ec

reference:
https://github.com/dotnet/aspnetcore/blob/3aa03f47c740a867a99fb7240509f988246bc01c/src/SignalR/clients/csharp/Http.Connections.Client/src/Internal/WebSocketsTransport.cs#L98-L105

[Copilot]

Pull Request Overview

This PR fixes an issue where setting the "User-Agent" header on .NET Framework would throw an exception.

• Implements conditional compilation to set an alternative header ("X-FeatBit-User-Agent") on Full Framework
• Includes clear inline comments referencing the SignalR client implementation as context

Comments suppressed due to low confidence (1)

src/FeatBit.ServerSdk/Transport/WebSocketTransport.cs:102

• Ensure that the use of the alternative header 'X-FeatBit-User-Agent' is supported by the server and is documented so that maintainers understand why a non-standard header is used in Full Framework builds.

webSocket.Options.SetRequestHeader("X-FeatBit-User-Agent", HttpConstants.UserAgent);