Add RpcTargetMetadata.FromShape<T> overloads that work on .NET Framework

[AArnott]

This involves some fancy syntax. Here's an explanation:

The preferred APIs are .NET-only. These are the generic methods with a type constraint of IShapeable<T>. But this interface has a static member, making it .NET-only.
But FromShape<T> is a convenient method to call, and fast too. We'd like to expose an overload of the method that doesn't have the generic type constraint, since we can implement it properly on .NET Framework. But C# does not let us declare two overloads that vary only by generic type constraint on the same type.

But we can declare such overloads when one is on an extension class instead. For instance methods, we could use ordinary extension method syntax. But since the methods to be overloaded are themselves static, we must use the new C# 14 extension members syntax, which permits extension static methods.

This allows multi-targeting code (such as our tests which I made a 1-line change to in order to exercise the new method) to simply use the preferred syntax and the compiler will automatically choose the best method based on the target framework.

The #if'd attributes on the new methods are to guide users when the compiler doesn't choose the preferred overload on .NET, since when that happens, it is an indication that the code will fail at runtime.
The RequiresDynamicCode attribute is a subtle requirement of PolyType only on .NET 8. .NET 9+ no longer has that requirement.

[Copilot]

Pull Request Overview

This PR adds .NET Framework-compatible overloads of RpcTargetMetadata.FromShape<T> methods using C# 14 extension static methods. The PR enables multi-targeting code to use the same convenient syntax across both .NET and .NET Framework, with the compiler automatically selecting the appropriate implementation based on the target framework.

• Adds extension static methods in a new RpcTargetMetadataExtensions class to provide .NET Framework-compatible overloads
• Updates target frameworks to include net9.0 for both the library and tests
• Adds RequiresUnreferencedCode attributes to classes using reflection in JsonMessageFormatter.cs

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/StreamJsonRpc/RpcTargetMetadataExtensions.cs	New file containing extension static methods for .NET Framework-compatible FromShape overloads
src/StreamJsonRpc/StreamJsonRpc.csproj	Adds net9.0 target framework and suppresses StyleCop SA1201 for new C# 14 extension syntax
src/StreamJsonRpc/JsonMessageFormatter.cs	Adds RequiresUnreferencedCode attributes to async enumerable converter classes
test/StreamJsonRpc.Tests/StreamJsonRpc.Tests.csproj	Adds net9.0 target framework
test/StreamJsonRpc.Tests/RpcTargetMetadataTests.cs	Updates test to use parameterless FromShape<T>() method, removing explicit provider argument

[etvorun]

[AArnott]

docfx is broken by this change. We'll have to block this change on dotnet/docfx#10808